It's common for a team to disband at the end of a season. Sometimes they want to retire on their winnings, sometimes the owner decides to found a new team, and sometimes the team appears to have only ever been set up as an elaborate expenses claim and was never expected to survive the season anyway. Some teams who do return decide to cut players from their old team, either because they were disappointed with their performance or because they can no longer afford to keep an expensive player on contract.
This whole process has a tendency to leave a lot of players kicking their heels, still capable of playing but not on a contract with any team participating in the season, so they make their services more generally available in hope of playing. Players who are not re-signed by their previous team enter free agency. The league commissioner will declare free agency open when he is satisfied with the number of teams signed up for the league and specify how long the period will last.
Teams of the same race, or capable of fielding that player race and type, can bid on a free agent's services. Some player types cross team types: for instance, Treemen will play for either Halfling or Wood Elf teams, but a Norse thrower will only play for Norse teams. If in doubt about whether a certain player can play for your team, check with the commissioner. In general, a player with the same cost, skills and development opportunities as a player on your team roster will be available.
Free agents might be unemployed, but they know how much their skills are worth, and will not be signed for less than their current player value (ignoring any award bonuses). At the end of the free agency period, the highest bidder gets the player. Any players not signed during this period will be absent until next season's off-season, having been forced to play for a minor league team in order to pay the bills.
Free agents may be signed from either active or defunct teams. Dead players and players confirmed retired may not be signed.
Players from orphaned teams may be signed, although they will not keep their full number of SPPs. The players have slacked off training in the absence of their coaches and so most of their SPPs have been lost between seasons. As such the appropriate number of SPPs for each player is that from the last updated roster, not the figures given in the record tables. The number given on the roster table should be accurate, but check with the relevant thread if in doubt.