I must echo Wagionvigil's sentiments. Checkout Geocaching.com or "google" GPS to get some background. These items like all electronics, vary from a very basic model (no maps, no electronic compass, black and white screen) up to a color screen with removable memory cards, downloads, etc. Prices also range from about $75.00 to $500 plus.
I don't know any that just have trail maps. Many of the mid to upper end units allow you to download maps. As far as the types of maps you can download, depends on the software you purchase. I know with my new Garmin 60Csx, I have an option to download topographic maps that are scaled either 1:100,000 or certain other topo maps that are 1:24,000.
Like all topo maps, the 1:24,000 will have much more detail on them than the 1:100,000 maps. This is where you will likely get your trail detail.
You can pick up an atlas of 1:100,000 topo maps for about $20 from most sport stores. If you want to get into the 1:24,000 you normally have to go the the USGS or find a map dealer in your area.
GPS units are a great tool, but you still need to have a detailed topo map of the area and a compass as well.