What are the differences between "ground stop", "ground delay", and "gate hold"?
@Pondlife has good quotes and I'll supply some of the direct knowledge he requested.

Ground Stop

Your destination airport cannot accommodate arrivals -- at all.  In this case, they will disallow all planes destined for that airport to take off from wherever they are.

Some examples of what might cause ground stops are:


Weather impacting standard arrival routes, probably more than one.  Aircraft are already in holding patterns and they are running low on altitudes/fixes to put us, and/or those of us in holds are getting close to diverting for fuel.
Airport or runway closure.  If you can't land, you probably shouldn't take off.
All gates are occupied.  If planes cannot depart, there may be nowhere to park arrivals.


You'll note that many times you'll see ground stops described as "Tier 1", "Tier 2".  In short, the T1 ARTCCs are those that directly border with the ARTCC of the destination.  T2 ARTCCs are those bordering the T1 ARTCCs and so on.  Sometimes only T1 departure airports are affected by a ground stop, and sometimes it is everyone.

Ground stops are issued with update times, usually (in my experience) 45-60 minutes later.  They are subject to cancellation at any time, or extension with updated in another 60 minutes.  

Ground Delays

These are like the Ground Stops but the airport knows it can accommodate you, just not exactly when you planned to be there.  The reason is most often weather at the destination airport.  Runway closures, accidents, construction, plowing/de-icing can all cause these delays also.   

An example

In good weather, airport A may land 90 aircraft per hour.  The helpful airlines have scheduled 110 airplanes to land in one particular hour (no slots at this airport!).  Low clouds and high winds have moved in and now airport A can only land 40 airplanes per hour.  Now of the 110 airplanes, 70 of them will be delayed.  The airlines and ATC will work out which 70 planes those are, and which 40 planes will be allowed in that hour, and start issuing ground delay times.  Of course, these delays spill over into the next hour and cascade.  

When a ground delay program is in effect, you will get a "Wheels up time" from ATC, which is when you have to be airborne.  Not sooner, and not much later (3-5 minutes usually).  Getting into your spot in the arrival flow depends on you taking off at this time.  In these scenarios ATC can figure out how/when to get you to your destination, if this isn't the case, that is usually when the Ground Stop program gets started.

Gate Hold Procedure

There is too much happening out on the taxiways, and no room for you.  Stay at the gate!

Reasons for gate holds:


De-icing is in effect and not enough room for everyone to line up at once.
The line to takeoff is too long
Your departure fixes are closed/saturated (This one happens a lot in the NYC Tracon)


In these cases, ATC will issue you a time to start and pushback via clearance delivery (at big airports) or ground control (smaller airports).  Sometimes ramp control can be helpful and help coordinate things... usually not.  When your time is up, you push and taxi and contact ground control.  Not any different than normal except you are given a specific time to do it.