What is the difference between infertility and sterility?

Strictily speaking the definitions currently accepted are:

Primary sterility: when the couple has not managed to achieve a pregnancy after having had unprotected intercourse for a year.
Secondary sterility: When , after having had a first child, the couple has not manged to achieve a second pregnancy after having had unprotected intercourse for two three years.

Primary infertility: When the couple achieves a pregnancy but is unable to maintain it long enough for it to lead to the birth of a baby.
Secondary infertility: When the couple , after the first pregnancy and labour, does not manage to maintain a pregnancy long enough for it to lead to the birth of a baby.