Oort Cloud 
Most of the comets lay at the distant reaches of our system in a hypothesized Oort cloud. At the very edge of the solar these comets orbit in very large loops around the distant reaches of our solar system. The passing of nearby stars, or other objects can alter their orbit, sending them speeding towards the inner reaches of our solar system. these comets typically retain very large orbits such that they will not return (once seen in the inner solar system) for many thousands of years.
Kuiper Belt 
Occasionally inteferences with the large gas giants of our solar system (jupiter and saturn) will further alter their orbit. These comets commonly roam in an area called the Kuiper belt, that surrounds the last planet Neptune. The orbits of these comets vary highly from the diminuitive comet Encke (3 years) to the famed comet Halley seen only once maybe twice a lifetime (76 years).
Still other comets have dangerously close orbits to the sun and dozens have been found in recent years by a solar observing spacecraft plummeting into our nearest star.
Observing them 
Irregardless of where comets come from or go. Comets are a regular visitor the inner solar system. Because comets cannot be detected at the furthest reaches of our solar system, there is always the chance for a dramatic visit by a truly well-traveled visitor. Amatuer astronomers have been enthusiastic searchers for these cosmic visitor since the beginning of time and according to astronomical convention, whomever first finds the comets, has their name immortalized in the heavens.
Why observe? 17P/Holmes 
On Oct 23, 2007, J. A. Henríquez Santana in the Canary Islands and Ramón Naves in Barcelona noticed an impressive spectacle, a bright and large comet gleemed through the bright full moon in the constellation Perseus. This was not a new comet, yet it was a supposedly boring one. The comet 17P Holmes was supposed to be a meagre 17th magnitude, far too dim for most telescopes to even detect in the bright moonlight.
The comet had suddenly brightened a half a million times and was easily seen by the unaided eye. Such is the importance of keeping an eye out for changes in a known comets. Within days, the most powerful telescopes on the earth (and the one above it, the hubble) would be viewing it.
So I make it a dialy ritual to check out and record as many details as I can about each viewable comet and like to be aware for new things while I'm at my telescope.
An extremely good web page for comets is Greg Crinklaw's Skyhound and by coincidence Holmes is coming out of the glare fo the sun. We should keep tabs on it!
Currently visible in a small scope (6" or less) 
Comets visible during October 2008 (place any observations below and include time, date, location, and any observable features)
17P/Holmes: A morning comet near Cancer visible in binoculars
- Very good to see as it has not been widely seen yet, its brightness and location might be useful to report
C/2008 A1 (McNaught): An evening comet in Ophiuchus visible in binoculars
C/2006 W3 (Christensen): A evening comet in Cassiopeia visible in small telescopes
- * Tough comet in a smalltown sky (5.0magn); I see a spherical 3' glow with little condensation and no nucleus.--Jolie (254mm reflector at 79X at Fri Oct 24,08 2:45UTC)
- * Hazy conditions, and much light pollution. --mikeu (16" f/10 SCT with CCD on 2008-11-22T02:53:23 UTC)
C/2006 OF2 (Broughton): A morning comet in Camelopardalis visible in small telescopes
- * Dim but definite in a smalltown sky (5.0magn); spherical with some condensation and a very faint nucleus. --Jolie (254mm reflector at 79X at Fri Oct 24,08 5:55UTC)
19P/Schwassmann-Wachmann: A morning comet in Cancer visible in small telescopes
- good to keep a watch on as it had a tendency for outbursts
C/2008 J1 (Boattini): A far-northern evening comet in Camelopardalis visible in a 6-inch telescope
205P/Giacobini: An evening comet in Aquarius visible in a 6-inch telescope
Wow, that is an impressively large list to be able to seen in a 6" or smaller telescope!--Jolie 16:38, 23 October 2008 (UTC)