Summer Tests are tests examined by the teachers in late May of the school year. They are generally examined to see how well you have done in the year of school. 1st, 2nd and 5th years have Summer tests (and some schools do TY Summer tests also - but most do not). They are consisted of everything you have generally done during the year but your teacher will give you a few hints of what is on the Summer tests of each subject including what chapters to study etc.
The Junior Cert is an exam set in June of 3rd year. It is compiled and examined by the State Examinations Commission. This exam prepares you for the Leaving Cert and helps you decide on your subjects or levels for Leaving Cert as well as Summer tests.