Rafiki -
Not all GPS units are alike, and so you really need to find the one that has the features you desire. There are two (maybe three, if we include marine) primary model types of GPS. Automotive, dash mount devices to get cars from point A to B, and have nice, big screens; and handheld, designed to be used afield with hikers and geocachers. They tend to be smaller and lighter. Mine clocks in around 6 oz.
They all can offer different features and different map sources, so I went with a model offered by a mapping company well known for it's data, Delorme. The unit comes with their Topo software, which provides me the ability to set routes to follow (it already has the I.R. trails and most National Parks already 'routed' on the maps) by setting start and stopping locations. The most recent model equivalent would be this:
http://www.rei.com/product/801770/delor ... -pn-60-gps
As far as some definitions, I'll do my best to keep it simple.
Topo Maps:
I believe refer primarily to different elevations. Correct, it's short for Topographic Map and it does just that. You get lines (typically every 10', but the map will say) showing you contours of the area, and then every 50 or 100' you get a 'major' contour for reference, and it's labelled. These maps often will indicate things like swamps/marshes, peak elevations, and reference points. Very common hiking fodder.
Orthoquads:
No Clue. Perhaps more accurately, Digital Orthophoto Quarter Quadrangle, or DOQQ. Basically, 1-meter resolution (1 meter=1 pixel) aerial images that have been rectified/corrected to eliminate distortion. Very often now in color.
NOAA Charts:
Yikes. Huh? National Oceanic and Atmospheric Administration "water map." These are the folks that put out the official waterway charts for large bodies of water, indicating obstructions, etc.
Aerial/Sat imagery:
Ummm....kinda like on google maps where you can zoom in and view objects and buildings surronding the street level? Or would this just be a helicopter view that is not as detailed? Somewhat like DOQQ, really, it's additional types of "looking down" images, of real life. Very handy if you're hiking backcountry, because you can often pick out things on the map. Various resolutions available, and zoomability is a function of the device, but very much like a Google Helicopter view. (But not "street view")
I should probably also mention USGS (US Geological Survey) "7.5 minute" quads, which often have Topo data on them. They are set to a specific scale (1:24000) which make them easy to use. These will also often show structures (at least in rural area) and other 'points of interest' that a cartographer may place.
I've never regretted buying a GPS, and have quite a bit of fun with it. I use it for hunting, hiking, and geocaching. (Geocaching is looking for small hidden items in various public places, and is a great way to get off trail at parks all around.) Bucket Bob was the one that turned me on to searching out the triangulation points embedded on Isle Royale, which is just another great reason to start hiking off trail, with purpose.
Smartphones will incorporate features of GPS, to some degree. The biggest achilles heal is that most require a data connection to maintain the mapping component, and you can't pop two AA batteries in your phone when it dies. They do just fine for Geocaching and automotive routing, but I'd stick to a more ruggedized and specific unit for hiking needs, especially in the data-void IR.