What is the difference between a one-tailed and two-tailed hypothesis?

A one-tailed hypothesis is directional, meaning that you expect your results to turn out a specific way. When constructing a one-tailed hypothesis, you are predicting the effect the independent variable will have on the dependent variable.

For example: "females will recall more words than males."

A two-tailed hypothesis is non-directional, meaning that you simply expect the independent variable to effect the dependent variable. The way (direction) in which it effects it is not specified.

For example: "there will be a significant difference between the number of words recalled by females and males" or "there will be a significant difference between A and B."