Speeding is driving faster than the law allows. It is not just "going fast" — it is going faster than the number on the sign for that road. So 60 km/h can be perfectly legal on one road and speeding on the next one, because the limit changed, not because the car changed. In this meaning the word is a noun, and it names the offence, the thing you can be punished for: you get a ticket for speeding, you get fined for speeding, the police stop you for speeding, a driver is charged with speeding. You will also see it in front of other words like an adjective — a speeding ticket, a speeding fine, a speeding offence — and all of those still point back to the same offence. Two things students find odd. First, it takes no article: we say "He was fined for speeding", never "for a speeding". Second, it is one of those crimes people admit to quite lightly. Someone will say "I was speeding a bit" the way they might say they were a few minutes late — it is against the law, but socially it sits far below drunk driving. Though actually, in careful or formal use the tone hardens fast: a news report saying a driver was speeding is pointing at the cause of a death. Separately, speeding is also just the -ing form of the verb speed, so you meet the same letters in "a speeding car" (a car going very fast, no law mentioned) or "the ambulance was speeding to the hospital". There the word is only about speed, not about breaking a rule. Which one you are looking at usually shows in the sentence: after "for", it is the offence; in front of a vehicle, it is usually just fast.