A term is the length of time a standard course lasts. The length of a term depends on the number of terms that a school offers courses. A term may be a semester (at a school that has two terms: fall and spring), a trimester, or quarter. Here is more on college terms.
Etymologically, semester means six months. (There is also the term trimester for three months, usually used in relation to pregnancy.) While a semester in a university is not necessarily six months long, it should be sufficiently close to that duration and therefore there can only be two semesters in a year.
A term is a more general term and it can be of varying duration. UK universities used to operate on the basis of three terms: an autumn (Michaelmas/Martinmas), spring (Lent/Epiphany/Hilary) and summer (Trinity/Easter). Some still do; others operate on a semester system. Some have both, where the spring term spilts up into the last bit of the first semester and the first bit of the second semester.