The user display often mimics devices that the user may already be familiar with which could fool a new user into thinking those devices are in the unit. For example a speedometer display matching the one on your car or a compass display that looks like a standard compass. Neither device is actually in a gps receiver but are computed based on your current position and doppler data available as part of that solution. So if you are moving the unit can look like it has these devices. It will also compute an altitude which causes some folks to wonder if there is an altimeter inside. Again this is a computed output and the unit does not have a built in altimeter. By the way, due to the satellite position the altimeter reading isn't very good and can be up to 30 meters off and swinging wildly. Both a compass and an altimeter are good backup devices for a gps when hiking.
Having said that, there is no reason why a unit couldn't have these extra sensors inside and indeed the Silva gps and some Garmin models do have a compass built in (and even an alitmeter) as do some survey grade receivers but in general inexpensive gps devices do not. A gps usually does have a built-in clock to help with its initial startup but after a position is computed the display of the time is based on data that comes from the satellites and not from the built-in clock. Using the satellite data for a clock provides you with a very accurate clock. Note that the behavior of these extra built-ins are different from the simulated displays in a standard gps. For example the altimeter does not need a fix to work but may be calibrated using a gps fix later. The compass can be used while the unit is stationary, unlike the simulated compass display, and even know which direction you are holding the unit in while the computed compass knows the direction the entire unit is moving in. Often these are entirely different things.
At one time all gps units attempted to cater to everyone. As units progressed and needs become more focus manufactures have begun to produce units that are specific to an application. While there are still universal units in the market place there are more and more specialized units available. For this reason you really need to understand exactly what you intend to do with a gps before you buy one. By the same token once you have a unit you will probably find uses for it that you didn't consider earlier so you will want a few features that are beyond the basic things you can currently think of. You may want to spend some time reading my short article on what can you do with a gps. You may also want to read Joe's and Jack's articles on what you really need in a gps based on application. They have What features do I need for hiking? and What features do I need for driving? If you need more technical background then I suggest Joe's site, Peter's site, and if you have a Macintosh then Karen's site. Each of those sites has references to other sites with specific data that you may find valuable.
A GPS receiver represents a quantum leap in technology over most items that you may have purchased. You need ot spend some time understanding the technology behind the gps system in order to determine if it will do what you need and to help you understand what to look for in your purchase.
First some recommendations of my own. Unless price is really an important consideration go for a 12 channel parallel unit. If you want the receiver to work without waiting around for 2 or more minutes with a clear view of the sky or to use it in applications where there is not always a clear view of the sky then get a 12 channel unit. Next, if you own a computer, then get one with a computer interface. Finally I would recommend a unit that can work standalone unless your application always needs to have a computer along. A standalone unit can work with a computer but a unit without a display won't work without one.
Here are a few things to help with your evaluation: I think the best thing you can do is to spend some time working with the proposed unit in your application. Or if that is impossible the see if you can try it out in the store. Familiarize yourself with the interface and decide which is more comfortable for you.
There are also some questions that you don't really need to ask:
Typical applications for gps include automotive, RVing, hiking, biking, aviation, motorcycles, and marine. Other categories include skiing, snowmobiling, and off-road driving. You need to decide how many of these you intend to use the unit for and what is really important. Many units have been specifically targeted at customers in specific categories. If you have specific mounting requirements then check into this as well.
A navigation receiver is not a surveying device but it is likely more accurate than most of the digital maps you use with it anyway. For more accuracy you can add a separate beacon receiver to supply correction data to the gps (some receivers have a beacon receiver built in but it still needs a separate antenna) or you might buy a unit that has WAAS capability. WAAS can provide corrections similar to a beacon receiver and uses the same antenna as your gps receiver. It, however, is line of site and does not offer as good a reception as a beacon receiver in areas covered by beacon receiver. In addition WAAS depends on geo stationary satellites which may be fairly low in the sky depending on your latitude so they may be blocked by buildings, trees, or hills. Both solutions can get your accuracy down to 3 meters or so, which is still not survey accuracy.
Most of all, enjoy your search. Welcome to the world of gps.
Still have questions? Post to news:sci.geo.satellite-nav or send them to me at: Dale DePriest