apply - When you want to apply a function to the rows or columns or both of a matrix and output is a one-dimensional if only row or column is selected else it is a 2D-matrix
lapply - When you want to apply a function to each element of a list in turn and get a list back.
sapply - When you want to apply a function to each element of a list in turn, but you want a vector back, rather than a list.
tapply - When you want to apply a function to subsets of a vector and the subsets are defined by some other vector, usually a factor.
