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C4 is explosive (bomb) used by Terrorists on de_* maps.
After planting it (3 seconds) it starts ticking for 45 seconds (35 for Competetive) when it can be defused by Counter-Terrorists (10 seconds or 5 seconds with defusal kit).

Of course for real bombs you would not want to stay around to keep the bomb secure for about 40 seconds (you would probably kill yourself with it or set the timer only to few seconds for you to run away). The fact is also that there is really low chance of someone defusing it. Police is often using controlled explosion to take care of it.
Warning for USA citizens: This should not encourage you to detonate bombs. It is just my realistic look at bombs (and bomb attacks).

If it is this stupid, why is it soo fun to play de_* maps?
There are (often) 2 places for the bomb to be placed on which causes CT to split and wait for T to go plant the bomb (when round time goes out, CT win).
It forces Terrorists to choose to which plant point/space they want to go/use. To do any action (kill CT team or plant the bomb).