The two sets of numbers representing your horizontal position may be given in Latitude/Longitude or UTM coordinates or some other system.  This is the grid.  It is also important to know the datum - what the numbers are relative to.

The GPS receiver manufacturers give different names to points depending on the purpose they serve.  There are waypoints, map points, route points, via points, track points and event markers to mention some common ones.

Some are essentially independent such as waypoints and map points.  Others are only defined when connected to other points such as route points and track points.  They define line segments.  Sometimes the independent ones can become connected points as when waypoints are used to form a route.

The GPSR gives you a point.  A compass gives you a direction.  If you have some reference points stored in the receiver, it can tell you the distance and direction to those points.  The GPSR can give you a direction if you move by using the point where you were with the point where you are now.  You should be moving about 2 mph or faster to get a good direction reading.

See  Garmin Screen Examples  for much more detail and some map information.
 

The GPS receiver can not receive any signals from a satellite "below" you so the geometry doesn't support an elevation value as accurate as it does a horizontal value.  By the same token if you are in a canyon, the geometry can support better accuracy along the axis parallel to the canyon than along the axis perpendicular to the canyon.

Some GPSR models also have a barometric altimeter in them.  Some brands and/or models will use the GPS signals to calibrate the barometric altimeter over time whereas others don't.  Whether the barometric elevation or the GPS elevation is shown, stored  in the track log, or stored with a waypoint depends on the brand, the model, and the firmware version.

The GPS elevation tends to have larger short-term errors than a barometric altimeter and is not affected by the weather in the long run.  The barometric altimeter tends to be fairly stable in the short-term but has larger long-term errors since it is affected by the weather.  Combining the two can be the best overall (early attempts at doing this weren't so good).

Some periods of the day are worse than other periods for getting a position fix.  The satellites rise about four minutes earlier each day so the same pattern will repeat the next day only four minutes earlier.  If you have buildings, trees, or mountains in the way of the satellites, then expect that there will be times when you can't get a location fix and other times when you can.

The signals from the satellites will go through some fabrics (non-metallic) such as a nylon day pack or clear glass.  They will not go through glass with a metallic film that is found on some vehicles or a half-inch of standing water.  The signals will "go through" rain.  Items that contain water, such as leaves, living beings, etc., will attenuate the signals if thin enough and block them completely if thick enough.

For a look at satellite availability, DOP (dilution of precision), overhead views:

Satellite Patterns

The highway screen is really for air and water navigation.  It gives those users a virtual "highway" to follow.  Land users have a real "highway" to follow.

From Bowditch, "American Practical Navigator":

Bearing (B, Brg.) is the direction of one terrestrial point from another, expressed as angular distance from 000° (North) clockwise through 360°.

Course (C, Cn) is the horizontal direction in which a vessel is steered or intended to be steered, expressed as angular distance from north clockwise through 360°. Strictly used, the term applies to direction through the water, not the direction intended to be made good over the ground.

Track made good (TMG) is the single resultant direction from the point of departure to point of arrival at any given time. Course of advance (COA) is the direction intended to be made good over the ground, and course over ground (COG) is the direction between a vessel’s last fix and an estimated position (EP). A course line is a line drawn on a chart extending in the direction of a course.

Heading (Hdg., SH) is the direction in which a vessel is pointed, expressed as angular distance from 000° clockwise through 360°. Do not confuse heading and course.  Heading constantly changes as a vessel yaws back and forth across the course due to sea, wind, and steering error.

Track (TR) is the intended horizontal direction of travel with respect to the earth. The terms intended track and trackline are used to indicate the path of intended travel. The track consists of one or a series of course lines, from the point of departure to the destination, along which it is intended to proceed.

A "mapping" receiver displays a background map (topographic, roads, etc.).  The others have a "blank" background.  Mapping receivers usually come with a built in basemap and can except downloaded maps from, in almost all cases, only the manufacturer's software. 

Only Magellan's MapSend™ can download maps to Magellan's receivers. Use Lowrance's MapCreate™ to download maps to Lowrance receivers. "Only" Garmin's MapSource™ can download maps to Garmin's receivers except that some people have reverse engineered Garmin's data structure and created GPS Mapper as one example.   Look for "GlobGPS", "MapDekode", and "GPSMapper" to create your own maps (a lot of work).

For GPS Mapper try:  http://gps.chrisb.org/gps_mapper.htm  and  http://www.gpsinformation.net/gpsmapper/gpsmapper.htm

For MapDekode try:  http://planeta.terra.com.br/informatica/download1/dekode_contract.htm

Also check the GPS TrackMaker® site for hints on map creation using MapDekode.

Magellan's structure has also been reverse engineered so some tools are also available for Magellan.

Most receivers let you turn logging ON and OFF.  When the storage area is full, the receiver might give you the option of either overwriting the oldest points that were stored or to stop logging track points. Not all models give you the option to stop, even from the same manufacturer.  For instance the Lowrance iFinder and Garmin eMap don't have an option to stop logging when the storage area is full.  They will overwrite the oldest points they recorded unless you stop them.

Most receivers support logging by time or distance intervals.  Garmin also has an "Auto" setting that logs a track point when you deviate from a straight line by a certain amount or speed drops to zero.  The Lowrance iFinder also has an "Auto" setting that logs a track point when you deviate from a straight line by a certain amount.  The "Auto" setting will probably give you the best representation of your route while using the fewest number of track points.  The Magellan SporTrak series has an "Auto" and an "Auto Detailed" setting.

Old receivers generally only record horizontal coordinates and not the elevation (altitude).  Newer receivers record three coordinates (x, y, and z) in both the waypoint mark and the track log.  So don't expect to get a graph of the altitude if track logging is off or if you have an old receiver.

The map accuracy circle shows the possible location in relationship to the map.  It includes inaccuracies of the base map or other loaded map and any distortions caused by the zoom levels on Garmin GPS receivers.  The accuracy circle is usually considerably larger than the accuracy value.  The accuracy value might be about a 50% confidence level not a 95% confidence level.  Different maps have different accuracys.