A proficiency test measures a learner's level of language. It can be compared with an achievement test, which evaluates a learner's understanding of specific material, a diagnostic test, which identify areas to work on, and a prognostic test, which tries to predict a learner's ability to complete a course or take an exam. Proficiency tests are uncommon within the classroom but very frequent as the end aim (and motivation) of language learning.
IELTS and TOEFL are examples of proficiency tests.
In the classroom
Proficiency tests often have a significant backwash effect on the classroom, as learners' focus narrows to preparing the test items. One way to make practice for exams more meaningful is by asking learners to prepare their own practice questions for the group.