A bombsite is an area in a map where the bomb must be planted in order for the Terrorists to win. There can be up to two bombsites in one map, each labelled A and B. Bombsites are only in Classic for Critical Strike Portable and Defuse for Critical Ops.
A bombsite is the only area where the C4 can be planted/activated, except in the case of Kamikazes in Critical Strike Portable, where they can activate it anywhere.
Usually, bombsites are located near CT spawn, presumably so the latter can guard it from incoming terrorists. Bombsites are marked in red circles with a cross inside them.