The Graduate Record Exam (GRE) is a standardized exam that features question types that closely reflect the kind of thinking you'll do in graduate or business school. Most veterinary schools require the GRE (although Purdue does not).
There are three sections on the GRE:
Verbal Reasoning — Measures the ability to analyze and draw conclusions from discourse and reason from incomplete data, understand multiple levels of meaning, such as literal, figurative and author’s intent, and summarize text and distinguish major from minor points, understand the meanings of words, sentences and entire texts, and understand relationships among words and among concepts. There is an emphasis on complex verbal reasoning skills.
Quantitative Reasoning — Measures the ability to understand, interpret and analyze quantitative information, solve problems using mathematical models, and apply the basic concepts of arithmetic, algebra, geometry and data analysis. There is an emphasis on quantitative reasoning skills.
Analytical Writing — Measures critical thinking and analytical writing skills, including the ability to articulate and support complex ideas with relevant reasons and examples, and examine claims and accompanying evidence. There is an emphasis on analytical writing skills.
The GRE offers a very useful guide on test content and structure.
You should prepare intensively by reviewing material in GRE prep books. GRE practice exams are also available in book and CD form to help you prepare. Khan Academy offers free topical videos that can assist in GRE preparation.