exams

what is the difference between the following exams..??
gre
cat
mat
sat
ielts
toefl
toeic

GRE = Graduate Record Examinations
CAT = Cognitive Ability Test
MAT = Management Aptitude Test
SAT = Standard Assessment Test
IELTS: International English Language Testing System
TOEFL: Test of English as a Foreign Language
TOEIC: Test of English for International Communication