It all depends on the league settings. In figuring out your particular league the relevant settings are scoring type, categories, roster changes, positions/bench spots/Max Games.
The biggest difference, in my opinion, between leagues is usually expressed in the scoring type. The two main scoring systems you'll see in FNBA are Rotisserie and Head 2 Head (some other FNBA formats will use "Point" scoring, but I don't believe this is offered by Yahoo! - please correct me if I am wrong).
ROTISSERIE Scoring: your team score is comprised of the sum of your rankings relative to the rest of the league across each individual category. The typical league will count 9 categories: FG% FT% 3PTM PTS REB AST ST BLK TO, some leagues choose not to count turnovers as often the most productive fantasy players are primary ball handlers and naturally get more turnovers. Let's make it easy and say you have a 10 team league with 9 categories. For each category, let's say PTS, all the teams in the league are ranked and given a score. So the team that has scored the most points across all of its active positions to that point in the season will get a 10, the next highest a 9, and so on. In a rotisserie league your score is cumulative throughout the season so really the only day that matters is the last day. Also point totals fluctuate a lot at the beginning of the season while teams separate themselves.
In a 10 team/9 category league, the most possible points a team could have would be 90. This would mean that they have the highest rankings in every statistical category (very unlikely). I am in two 10-team rotisserie leagues this season, and at this early point in the scores in both leagues range from the mid-20s to high 60s with the majority of the players around the 50s.
So, in essence, a rotisserie league has each team simulatenously competing against all the other teams in every individual category throughout the entire season. However if you were in first place through the entire season but somehow you fall into second on the last day, you still finish in second.
HEAD TO HEAD: H2H teams will be more familiar to Fantasy Footballers. In this format your team is pitted against one opponent's team for an entire week. Your team's total contributions in each category are compared and you receive one point for each category you win. The categories won to categories lost ratio essentially becomes your team record for the season. Each week you go 9-0 (outperform your opponent in every category) or 0-9 (the opposite) or anywhere in between. At the end of the season the team with the most wins is the victor.
Either Rotisserie or Head to Head formats can be played with a "playoff" structure in the final weeks of the season where the top teams compete for victory. Personally I've never done playoffs with Rotisserie, although I believe it is possible (please correct me if I am mistaken) however I've found it to be a fun twist at the end of a H2H season.
CATEGORIES: It is important to know what categories your league is counting and try to pick up players who will contribute across multiple categories without seriously depriving you in any 1 category. You may find a player who gets 12 rebounds a game but scores 2 points and shoots a terrible FT%, this would not be as attractive a player as one who has a more even line (again, just my opinion).
ROSTER CHANGES: Leagues have either daily or weekly roster changes, with head to head leagues usually being weekly and rotisserie going either way. This is largely a matter of preference for the league.
Weekly means is that your lineup will only be altered one day a week, typically Monday or Sunday. So what ever players you have starting on Monday will be your starters until at least the next monday. This more closely simulates a real team in that you won't get production every day since most teams have only 3 or 4 games a week. The disadvantage here is that if a player gets injured or traded or demoted you may be stuck with a unproductive player for an entire week while a perfectly good option languishes on your bench. (Please note that you can still access your team and make changes at any time in a weekly league, however no changes will take affect until the next week)
Daily means that you can change your roster as often or rarely as you like. This provides more flexibility in matchups and lineup changes, however some players feel that it unfairly burdens the more active players.
ROSTER SPOTS: Obviously the number of position players and bench spots is relevant to your scoring. The standard Yahoo! league has the following postions: PG, SG, G, SF, PF, F, C, C, Util, Util, B, B, B. The G and F positions can be filled by any guard or forward respectively and Util can be a player of any position. The NBA changed the rules on the injured list before last season and FNBA followed suit so there are generally no IL spots on an FNBA roster. For each position you can only start a player 82 games for the entire season (the number of games in a real NBA season). Once you have started a player at PG 82 times no further statistical contribution will be counted from that position.
I hope this helped to answer your question!