NFL Draft Eligibility

The draft is the first chance teams get to conduct contract negotiations with players who have been out of high school for three years.

Most drafted players come directly out of NCAA programs as seniors or juniors, all players entering the draft must at least be in their junior year, while some players are selected from other professional leagues. All players who choose to enter the selected year's draft must sign an agent prior to entering the draft. A player who is drafted but does not sign a contract can sit out that season, which is referred to as a "holdout", and can re-enter the draft the following year unless they were told differently by the NFL commissioner.

source: http://en.wikipedia.org/wiki/NFL_Draft)

Tests/evaluations include:

40 yard dash
Bench press
Vertical jump
Broad jump
20-yard shuttle
Three-cone drill
60-yard shuttle
Position-specific drills
Interviews
Physical measurements
Injury evaluation
Drug screen
The Cybex Test
The Wonderlic Test