A graduate assistant is a person who serves in a support role (assistantship) at a university, usually while completing post-graduate education. The individual typically assists professors with instructional responsibilities as teaching assistants or with academic research responsibilities as research assistants, aids coaches with an athletic team, or is employed by other university departments (such as housing or academic advising) in an entry-level capacity.
Rather than receive hourly wages, GAs are often remunerated in the form of a stipend. Assistantships provide much needed experience for graduate students, increasing their future employment options.
A stipend is defined as a fixed sum of money paid periodically for services or to defray expenses. That remuneration is termed a "fee" or "stipend" rather than salary or wages is immaterial. The stipend allows for the graduate student to focus 100% on their studies instead of a full-time job, but pays just the same as if they were working. A PhD graduate stipend is considered income in the United States.