[Mark Rejhon]

Important:
"Assisted GPS" is not the same as "Antenna GPS".

Here's an article.

This is correct, but let's be clear there's some confusion:
Some people confuse "aGPS" as "Antenna GPS" versus "Assisted GPS".
Also you should call "true" GPS as "native-only GPS" because Assisted GPS is also true GPS. Some people call antenna triangulation as "Antenna GPS". This is incorrect. Sometimes people mix up "Antenna GPS" and "Assisted GPS" (The AGPS acronym) or make them the same thing. That is not true. One is true GPS and the other is not.
Let me clear up confusion with Plain English<tm>.
I'm writing this from scratch, here goes...
Quality Scale
OKAY: Antenna Triangulation or WiFi Location (Incorrectly called Antenna GPS)
GOOD: Native GPS
BETTER: Assisted GPS (except the Verizon type where you pay extra)
BEST: Assisted GPS with fallback to Native GPS and Antenna Triangulation
Lock Speed And Accuracy Scale
ANTENNA LOCATION - fast lock, less accurate, works indoors
WiFi LOCATION - fast lock, less accurate, works indoors
NATIVE GPS - slow lock, more accurate, outdoors only
ASSISTED GPS - medium-fast lock, more accurate, outdoors only
Difference between Native GPS and Assisted GPS
NATIVE GPS Can lock without phone reception, takes one minute to lock because it needs to search for satellites harder, slow lock, works badly indoors.
ASSISTED GPS Downloads information over the network on how to find the GPS satellites more easily, locks much faster if you have a 3G signal, works slightly better indoors in 1-level and 2-level buildings that aren't metal.
What is Satellite Ephemeris?
Native GPS downloads ephemeris directly from the GPS satellite to help it find the location of GPS satellites.
Assisted GPS downloads ephemeris over EDGE or 3G from the network, to help it find the location of GPS satellites.
Ephemeris is mathematical data to help the cellphone calculate its position. As a grossly over-simplified Math 101 or Excel Macro 101 example, you need to calculate
"Location = ExecuteFormula(A, B)"
where A is the location of the satellites and B is the doppler effect it is detecting in the satellite signal. (You've heard of doppler radar on weather forecasts.) It's possible to download A over the Internet instead of getting A directly from the satellites. All kinds of GPS always needs to get B from the satellite signal, however A doesn't need to be obtained from the satellite signal.
Native GPS can do A and B directly from satellite signal
Assisted GPS can only do B from the satellite signal, and download A over cellphone network.
What is Doppler Effect?
Doppler is the frequency/pitch change in signal from object moving towards or away from you. A train or car going towards you always sounds higher-pitched than when it is moving away from you. The phone listens to the varying pitch in the radio signal being transmitted by the moving GPS satellites, and uses that information to help calculate position.

...

Sometimes GPS can lock indoors in single-level and two-level buildings. For this, Assisted GPS works better than Native GPS because the location signal is easier to receive than the ephemeris signal (which can be downloaded off the Internet instead). Assisted GPS needs to download approximately 100 kilobytes of data for one day's worth of GPS lock capability outside cellphone reception areas. Your cellphone will need to download several day's advance of satellite ephemeris in order to gain GPS lock capability for extended time periods outside cellphone reception areas, if the GPS chip inside your cellphone has the inability to download ephemeris
The best cellphone software can do all the above simultaneously, for maximum speed and maximum accuracy. Locks onto antennas and WiFi, followed by GPS once satellite lock is achieved. My BlackBerry can only do two of the above, but I notice its behaviour: When I first launch Google Maps, it gives me a 1000-meter approximation immediately. A few seconds to a minute later, it improves to 3-meter accuracy as it achieves GPS lock outdoors.
Anything called "GPS" should be able to receive the satellite signal directly. Antenna Triangulation does not, while Assisted GPS does.. Native GPS is best if you're outside of reception, my BlackBerry Curve can lock onto a GPS signal even without a Rogers signal. Some Assisted GPS requires both the GPS signal and cellphone reception available simultaneously, so sometimes "GOOD" and "BETTER" needs to be swapped around for some people. Also, once Assisted GPS has locked on, you can safely go outside of reception, it's just the first lock that may not work on cellphones outside reception that only has Assisted GPS if it has the inability to download satellite ephemeris [wikipedia] over the GPS airwaves. More technical information found on Wikipedia and gpspassion.com
Devices with multiple location capabilities, can locate through multiple sources for maximum speed and accuracy. Less inaccuracy, less waiting for lock.
Sources:

• Global Positioning System on Wikipedia
• Assisted GPS on Wikipedia
• HowStuffWorks.com on GPS
• Ephemeris on Wikipedia
• Doppler Effect on Wikipedia