Generally, teams like to keep young players on the practice squad so they can develop without taking up spots on the active roster. Undrafted rookie free-agents are popular along with late-round draft picks. A player can be eligible if he does not have an accrued season in the NFL. To have an accrued season, you have to be active for at least six regular-season games.
A player can also go to the practice squad with an accrued season provided they were on the 45-man active roster for less than nine-regular season games.