What is ground speed versus air speed and why is the difference important? Howdo you compute one if you have the other?

If you are moving by Air or by Water, Currents and winds will affect your "Speed over ground", It is what a GPS will tell you,
If you are going 60 Miles per hour in a 30 mile an hour head wind, a GPS will tell you you are moving 30 MPH of Speed made Good, or Speed over ground(Civilian) This may mean you need twice as much fuel, kind of Important.
With No Islands in sight, it can SXXK
It will help in determining your arrival. VERY important.

Stratmando is correct. Air speed is measured relative the air the plane is moving in; ground speed merely measures the resulting actual velocity over the ground. Also consider that if you try to take off with a 30 MPH tail wind and your plane requires a 60 MPH air speed to gain enough lift to fly, you will have a 90 MPH ground speed before you have the lift needed to fly. It's tough on landing gear and, on a short runway, you may run out of runway before lifting off. That is why airplanes always stake off facing the wind, or as nearly so as possible depending on available runway alignment. A similar issue is present on landing, so airplanes always land heading into the wind, or a close as possible.

I don't know what is the difference between air speed and ground speed! like in kn or something

Hopefully the 2 above post will explain.
You compare both at Miles Per Hour OR
Kph.

TANNY242 - THE two of them explained it very well; I will try:: IF you weere in a air plane and you had a air speed indicated on the air speed insturment as being indicated 200 mph, OK now say you have a head wind (WIND BLOWING AGAINST THE FRONT AF YOUR PLANE) AT SAY 30 MPH, YOUR GROUND SPEED OVER THE GROUND <WOULD BE (200MPH MINUS 30 MPH =170 MPH OVER$ THE GROUND:: NOW IF IT was turned around and it was atail wind you would add the 30mph to the 200mph, now that would make your travel over the gropund 230 mph:: NOW IF YOU WANT TO CHANGE MPH TO KNOTS ( MULTIPLY THE MPH x! 8 to = knotsIF YOU HAVE ANY MORE TROUBLE ON THIS QUESTION JUST COME BACK TO THIS POST<
AND HOLLER AND ONE OF US PILOTS WILL TRY TO HELP YOU OUT: HAVE A GOOD DAY AND GOD BLESS:: F.B.E. (EAGLE 1 out)

see it like this... wind speed is speed of air with respect to ground and airspeed is speed of aircraft with respect to the wind and ground speed is speed of aircraft with respect to ground... so, ground speed=vector sum of airspeed and wind speed.
if airspeed=60 kph and there is a tail wind of 40 kph, it means that aircraft is moving through the air with a speed of 60 kph... but the air is also moving in the same direction with a speed of 40 kph... so the aircraft would have to move faster to catch up with the wind and maintain an airspeed of 60 kph, thus increasing its ground speed...

its so simple'
Airspeed=ground speed (+ --) wind speed
let suppose air craft speed is 5 km/hr on the ground in the direction of the wind and wind speed is 2 km/hr the result will be
air speed= 5+2=7 km/hr
similarly if the air craft is moving in opposite direction of the wind,the result will be
air speed=5--2=3 km/hr
simply if the moving body is in the same direction of the wind we should add the wind speed or in opposite direction we should subtract the wind speed.

Another way to look at it is: If you were standing still in a 20 mph wind, your ground speed is zero but you do have a wind speed of 20 mph. Technically, an aircraft can take off and fly and not get anywhere fast. An aircraft that flies slowly say at 40 mph flies into a 40 mpg wind and is "still" over the ground. This is one reason why planes parked at an airport need to be tied to the ground. If you're still not confused: air's speed is measured in ground speed but airspeed and ground speed are only equal when the wind vector is zero.