A deep league is a league with more than the standard number of teams/owners. Whereas standard leagues tend to have 12 teams, deep leagues can have anywhere from 14-16 teams.
Deep league draft strategies are much different than standard leagues. In standard the leagues, the goal is to draft steady players that consistently contribute to the team's overall score each week. Due to the additional number of teams in deep leagues, there tend to be less steady contributors available. Thus, managers must turn their focus to "boom or bust" players. The term "boom or bust" refers to highly volatile players that can either erupt for 20 points or provide little to no scoring. Finding success in a deep league often comes down to finding hidden value in players that are not reliable starters but have the potential to excel in the right matchups.
Drafting "handcuffs" is also crucial in order to achieve success in a deep league. Handcuffs are players that are currently real-life backups, but are likely to slide into a starting role should the regular starter sustain an injury or underperform. Finding talented players that simply haven't had a chance in the spotlight requires an extensive amount of research and good fortune, but can ultimately pay off in a big way.
In addition to having more teams, deep leagues also have expanded roster slots. For example, whereas a standard league typically requires the owner to start only one quarterback each week, deep leagues often have two quarterback slots in the starting lineup.
The number of players fantasy managers are permitted to carry on their bench is also increased, giving fantasy owners a chance to stash several players in hopes that one will have a breakthrough season.
Deep leagues are generally much more challenging than standard leagues, due to the increased number of fantasy managers vying for the same players. It is for this reason that deep leagues are mostly occupied by experienced fantasy football players that are willing to invest extra time and effort in managing their teams.