The International English Language Testing System or IELTS is a standardized English proficiency test taken for pursuing higher education in English-speaking countries.
TOEFL is a standardized English Language proficiency test that evaluates your command over reading, listening, speaking and writing skills to perform academic tasks at the university level.
The Graduate Record Examination-General Test or GRE is a standardized, computer-delivered test that is an admissions requirement for pursuing masters (MS), doctoral (PhD) and now even MBA programs in many graduate schools around the world.
SAT is a standardized paper-based test that is an admissions requirement for most undergraduate programs of colleges in US.
The GMAT is a standardized, computer-adaptive test that lets you showcase your writing, reasoning and analytical skills in order to pursue an MBA from a graduate school of business.