Why is the complexity of the C++ STL map container O(log(n))?

For C++ STL containers such as vector and list, the complexity of finding elements and inserting or removing them is self-explanatory. However, for the map container, even though I know from my reading that the access and insertion complexity/performance is O(log(n)), I can't work out why. I clearly don't understand maps as much as I need to, so some enlightenment on this topic would be very much appreciated.