IAS is your indicated air speed. This is purely the speed the air is travelling past your aircraft, relatively speaking. (even sitting on the ground, with a head wind of, say, 50 knots, the IAS will be 50.)
GS is you actual speed over the ground. For example, if you're travelling at 60 knots into a headwind of 60 knots, your ground speed will be a big fat 0. You'll be hovering.
Mach speed is very important. This is your speed relative to the speed of sound. At mach 0.5 you are traveling at half the speed of sound. At mach 1 you are travelling exactly at the speed of sound. (although the airflow at certain parts may be travelling faster or slower around you, depending on curves, incipient shock waves and all that.)
Mach speed is dependent on pressure and altitude.
Mach is also varied by temp. Higher temp means higher speed of sound.
Knots are an old nautical term. Not sure why they're still used, but a nautical mile is one minute of longditude. A very clever thing to derive a unit of distance from.
I hope this helps. Maybe others can put it clearer, or more correct.
ps I was assuming GS was ground speed, not Glide Slope!!
