DSOs are of the following types: Galaxy(G), Nebula(N), Planetary Nebula(PN), Supernova Remnant(SNR), Globular Cluster(GC), Open Cluster(OC).
Galaxies are vast systems of stars, gas and dust, and are outside of the Milky Way Galaxy where our solar system resides. The galaxies can be divided into several classes which include the Spiral, Barred Spiral and Elliptical varieties. The main classes are also divided into sub-types. Our own Milky Way Galaxy is of the spiral class. Most of the galaxies on the Best DSOs list are of the spiral variety. M31, also known as the Andromeda Galaxy in the constellation Andromeda (photo below) is the closest spiral Galaxy to us. It is a magnificent sight in binoculars or a small telescope. Due to its large angular size, it requires a low power field of view to frame the whole object. The pair of galaxies M81 and M82 in Ursa Major can also be seen together in a low power field of view. Impressive and not to be missed.
Nebulas are large clouds of gas and dust, mostly hydrogen but with the other elements in varying smaller amounts. They can be of the bright or dark variety. The nebulas are where the stars are created as the nebula contracts and heats up under its own gravitational weight. M42, the Orion Nebula in Orion, is the best known bright Nebula and a magnificent sight in any telescope. An 8 inch or larger instrument is recommended to bring out the great amount of detail presented by this object. At higher power you can observe the Trapezium, a cluster of 4 extremely hot newly born stars that emit the ultraviolet light which causes the nebula to glow. M8 (Lagoon Nebula) and M20 (Triffid Nebula) in Sagittarius are a pair of worthy targets which can be seen together in a binocular field of view. A low power telescope field of view will show more detail in each nebula viewed separately. NGC 3372, the eta carinae nebula in the southern hemisphere constallation Carina is very large and bright and a beautiful sight.
Planetary Nebulas have nothing to do with planets. The were given that label by early observers who thought these objects had a similar appearance to the newly discovered planet Uranus. Their true nature was later discovered to be the blown off outer layers of stars that have used up their supply of the lighter elements needed for fusion reactions to continue. The cores of these dying stars survive in the form of extremely hot, ultraviolet light emitting white dwarf stars, which cause the blown off outer shell to glow. Prominent examples of these objects include M57 (Ring Nebula) in Lyra and M27 (Dumbell Nebula) in Vulpecula. Use middle to high power on these objects, since they have small angular size.
Supernova Remnants are the remains of the cataclysmic explosions of very massive stars that have gone through several cycles of fusion reactions, combining lighter elements into ever heavier elements, ending with the production of iron. Beyond iron, the continuation of the fusion cycle can not continue. The star suddenly collapses under its immense gravitational weight, no longer able to balance this contracting force with outward pressure from fusion. At this point the star explodes with a tremendous output of energy so great that for a brief time, the exploding star becomes as bright as an entire galaxy of billions of suns. In this immense explosion the heaviest elements in the universe are forged, up to Uranium. Only some man-made elements like plutonium are heavier, but they are very unstable. The core that is left behind after the explosion is extremely massive but concentrated within a relatively tiny volume. This core ends up as a nutron star or in some cases a black hole, due to the extreme curvature of space-time at its surface. In the case of the black hole, no object actually continues to exist, but only a deep well of gravity, where time appears to stop relative to the outside universe beyond the event horizon. A few supernova remnants are on the Best DSOs list. These include M1 (Crab Nebula) in Taurus and NGC 6960/6992 in Cygnus. They can be seen in an 8 inch telescope. The best contrast will be seen with the aid of a narrow band interference filter or line filter screwed into the eyepiece. These filters let only a tiny range of light frequencies pass through, thereby darkening the background sky and making the object more prominent in comparison.
Globular Clusters are big balls of stars packed more closely than usual. They exist in a halo around the galaxy, mostly concentrated around the central hub of the galaxy. The larges ones have up to about a half million to a million stars. The Milky Way Galaxy has in excess of a hundred of these objects. They are observed around other galaxies as well. The two biggest and best Globulars are NGC 5139 in Centaurus and NGC 104 in Tucana, both in the southern hemisphere. For northern observers, look for M13 in Hercules, M22 in Sagittarius, M3 in Canes Venatici and M5 in Serpens. M13 is usually favored since it appears overhead at mid-northern latitudes. It is best to observe objects as high above the horizon as possible since then you are looking thru the least amount of atmosphere. Objects near the horizon suffer from atmospheric extinction, diminishing the view. However, even though M22 is much closer to the horizon for northern observers, it is easier to resolve than M13.
Open Clusters are star clusters consisting of from a few stars to several hundred stars. They are almost exclusivly located in the spiral arms of galaxies. The best known object of this type is M45 (Pleiades) in Taurus. M45 has a relatively large angular size, so it is best observed in binoculars. It is very bright and impressive in small aperture instruments at low power. NGC 869/884 is the famous Double Cluster in Perseus. Both clusters can be seen together in a low power telescope field. The two clusters balance each other beautifully and provide a spetacular view. It is my favorite cluster. Other prominent clusters in the north include M35 in Gemini, M37 in Auriga, M41 in Canes Major, M44 in Cancer and M11 in Scutum. In the southern hemisphere look for NGC 3532 and IC 2602 in Carina, NGC 4755 in Crux, and M6, M7 and NGC 6231 in Scorpious
By now yo will recognize that all of the objects in the Best DSOs list are from two catalogues. The objects with M prefixes are from the famous Messier catalogue, which consists of 110 objects. It was compiled in the eighteenth century by Charles Messier, a French comet observor and discoverer. This is the first known compilation of non-stellar dark sky objects. With the exception of some star clusters, Messier probably had no concept of the nature of these objects. Messier made his observations from Paris, so he could not observe Objects deep in the southern hemisphere. The one big mystery is why the Double cluster in Perseus (NGC869/884), which is far into the northern sky and easily visible from his latitude, did not make it onto his list. The objects with NGC prefixes are from the New General Catalogue compiled by JLE Dreyer in the late nineteenth century based mostly on the observations of William and John Herschel. The NGC contains just short of 8,000 objects. The Index Catalogues (IC prefixes) are a supplement to the NGC. Most of the IC objects are faint and best observed with large telescopes. The true nature of most DSOs were not realized until the twentieth century.
If you are a beginner in astronomy, I would recommend learning the zero and first magnitude stars, the most prominent constellations and the movement of these accross the sky during the course of the night and through the various times of the year. Obtain a good star atlas. They are available on the internet from Sky & Telesope and other vendors. Star charts are also available as applications for computers, tablets and phones. Of course it is possible to buy advanced telescopes with computer driven catalogues of thousands of sky objects that can easily be set up to slew automatically to any object desired. These do have their place, but when it comes to astronomy I am old school. There is a great deal of satisfaction to be had by being able to look up into the night sky, immediately identify the brighter stars and constellations of the season, and to know at a glance which DSOs are currently viewable, and then be able to point your instrument at them manually, either from memory or by reference to a star atlas. It seems like the hard way at the beginning, but as with many activities, you receive benefits based on the effort you put in. Enjoy and Happy Observing!