Complexity Classes
The following list contains common time complexities of algorithms:

O(1) The running time of a constanttime algorithm does not depend on the input size. A typical constanttime algorithm is a direct formula that calculates the answer.

O($\log n$) A logarithmic algorithm often halves the input size at each step. The running time of such an algorithm is logarithmic, because $\log n$ equals the number of times n must be divided by 2 to get 1.

O($\sqrt{n}$) A square root algorithm is slower than O($\log n$) but faster than O(n). A special property of square roots is that $\sqrt{n} = n / \sqrt{n}$, so the square root $\sqrt{n}$ lies, in some sense, in the middle of the input.

O(n) Alinear algorithm goes through the input a constant number of times.This is often the best possible time complexity, because it is usually necessary to access each input element at least once before reporting the answer.

O($n\log n$) This time complexity often indicates that the algorithm sorts the input, because the time complexity of efficient sorting algorithms is O(nlogn). Another possibility is that the algorithm uses a data structure where each operation takes O($\log n$) time.

O($n^2$) A quadratic algorithm often contains two nested loops. It is possible to go through all pairs of the input elements in O($n^2$) time.

O($n^3$) A cubic algorithm often contains three nested loops. It is possible to go through all triplets of the input elements in O($n^3$) time.

O($2^n$) This time complexity often indicates that the algorithm iterates through all subsets of the input elements. For example, the subsets of {1,2,3} are $\varnothing$, {1}, {2}, {3}, {1,2}, {1,3}, {2,3} and {1,2,3}.

O(n!) This time complexity often indicates that the algorithm iterates through all permutations of the input elements. For example, the permutations of {1,2,3} are (1,2,3), (1,3,2), (2,1,3), (2,3,1), (3,1,2) and (3,2,1).
An algorithm is polynomial if its time complexity is at most O($n^k$) where k is a constant. All the above time complexities except O($2^n$) and O(n!) are polynomial. In practice, the constant k is usually small, and therefore a polynomial time complexity roughly means that the algorithm is efficient.