^ It's basically a concise and very rough approximation of the growth rate of a function.
< For instance, n, 100n, and n + 50000 are all in O(n) because they increase linearly; while anything with an n^2 and no n^3, 2^n, n!, or anything else that increases faster is in O(n^2).
v It's often used to classify the efficiency of computer algorithms when you really only care about how long it'll take to run when you're using arbitrarily large inputs. Quicksort and mergesort, for instance, both run in O(n log(n)) time and as such are pretty equivalent. One may take, say, ten times as long to run in some particular implementation, which is a significant difference, but nothing compared to bubblesort, which runs in O(n^2) time.