Originally, the word "nebula" referred to almost any extended astronomical object (other than planets and comets). The word "nebula" originally meant "cloud", turns out that these "clouds" were gas clouds lit by neighboring stars or from stars inside the cloud itself. There are five types of gaseous nebulas, they are: 

                    Planetary

                    Emission

                    Reflection

                    Dark

                    Supernovae

A brief description and accompanying photograph is shown below.

Planetary Nebulae

Planetary nebulae are shells of gas thrown out by stars nearing the end of their lives and are mainly circular in appearance. They're called planetary because of their circular shape as seen when you view them through a low powered telescope. Its the star at the center of the Planetary nebula that makes the gas cloud shine. Planetary nebulae can sometimes look like a doughnut or a ring, an example of this is the ring nebula (M57) that's in the Constellation Lyra. A typical planetary nebula is less than one light-year across. The photograph shows the Dumbbell Nebula M27 in Vulpecula.

Emission Nebulae (Type of a Diffuse Nebula)

Emission nebulae are clouds of high temperature gas. The atoms in the cloud are energized by ultraviolet light from a nearby star and emit radiation as they fall back into lower energy states (in much the same way as a neon light). These nebulae are usually red because the predominant emission line of hydrogen happens to be red (other colors are produced by other atoms, but hydrogen is by far the most abundant). Emission nebulae are usually the sites of recent and ongoing star formation.  An example of an emission nebula is the Great Nebula in the constellation of Orion. Some nebulae are not as bright as others.

Reflection Nebulae ( Another type of diffuse nebula)

Reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. Reflection nebulae are usually sites of star formation. They're usually blue in colour because light scattering is more efficient for blue light. Reflection nebulae and emission nebulae are often seen together and are sometimes referred to as diffuse nebulae. An example of this type, which is called a reflection nebula, is the nebula in the Pleiades star cluster. You'll need to take a long exposure photograph to see this nebula!

Dark Nebulae

Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. They are physically very similar to reflection nebulae; they look different only because of the geometry of the light source, the cloud and the Earth. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is a few hundred light-years across.

Supernovae

Supernovae occur when a massive star ends its life in an violent blaze of glory. For a few days a supernova emits as much energy as a whole galaxy. When it's all over, a large fraction of the star is blown into space as a supernova remnant. A typical supernova remnant is at most few light-years across. An example of this is the Crab Nebula (M1) in the constellation Taurus.

Nebulae are very difficult objects to see with a telescope. The only one I've managed to track down is the Orion Nebula (M42). Others like the Crab Nebula (M1 - Taurus) and the Ring Nebula (M57 - Lyra) have proved to be very elusive (ttechy speak for I can't see / find them). I'll continue to search for them and post my results.

Update: 

Well using my detailed star charts I managed to find both M57 (the Ring Nebula) and M27 (the Dumbbell nebula). The main reason I couldn't find M57 before was due to its small size. I had looked at it before but didn't realise I was doing so! Increasing magnification using the 9mm eyepiece made all the difference. M27 is much larger in size but I needed a star chart to help me locate it. Once I had it was visible in the 25mm eyepiece. Both nebula looked great and stood up well to high power magnification. Check out the observing report here. Can't wait for winter and the return of Taurus as I've never managed to track down M1, the Crab nebula either - this time though.......

November 2000:

I've finally managed to track down the Crab Nebula in Taurus (M1). I used a detailed star chart to help navigate from Zeta following small groupings of faint stars. I now had the location of M1 in the center of my eyepiece. I could just barely make out the faint gray shape of the nebula - it was incredibly difficult to see (there was no Moon and Taurus wasn't very high). I used a Sky Glow filter (borrowed from a friend) to see if it would make a difference. It did. The nebula was easier to see though it didn't give up any detail in its structure. Perhaps with a darker sky and the constellation higher up the nebula would show up better. At least I've found it though!

The same night I also used the filter to check out the Orion nebula. WOW. What a sight that was. The nebula filled the whole eyepiece (32mm Sirius Plossl) with filaments arcing way up from the trapezium area. I spent ages just taking in the sight. Again it was low in the sky and I think my view can only get better - can't wait.

You can check the observing report here.

The Picture on the left is a fair representation of the image you'll see of the Orion Nebula using a 6" reflector and a 9mm eyepiece. (You may see a little less depending on the level of light pollution in your area). On a clear night you can make out the structure of the dust cloud quite easily and the four stars of the Trapezium can be made out. The faintest is visible (at magnitude 6.1) but you have to look for it in the glare of the other stars and the dust cloud of the nebula. This is well worth revisiting as the appearance of the nebula changes each time you look at it - probably down to the conditions on the night.