OK very generally
PG - Point Guard - meant to create for your other players. Often brings the ball up to start an attack but primary role is creating assists in most teams.
Key skill Passing.
Some PGs have good jump shot and jump range so they can score if no open pass. Others have driving and inside scoring for same reason (but on a team looking inside not outside). Others may have good outside defense to shut down opposing guards shots and to get interceptions from opponents passes.
SG - Shooting Guard - The other guard is as the name suggests primarily responsible for scoring.
Key skill would be Jump shot.
Hopefully has decent Jump shot and jump range for outside shots but also driving and inside scoring so can score in many ways. Defense can be usefull as they often match up against an opponents key scorer, while passing or rebounding will help the team out generally in those areas.
SF - Small Forward (some call it shooting forward though) - This position often impacts on the overall style of your team as the other 4 positions generally are split into a couple of types of player in real life where-as there are so many types of SF styles. A SF specialist could be an outside scorer or inside scorer on a team who concentrate on that type of tactic or could be a defender and rebounder. Sometimes rather than playing to the style of the team the SF will make up for a lack of a certain skill in the rest of the team. Others are all-round players who can do a bit of everything.
A player with similar skills to shooting guards is common where these 2 players act as wings or swingmen. Other SFs are excelent rebounders and blockers but less athletic but still capable of scoring inside. These teams would play with essentially 3 big men and try to dominate inside, though perhaps their SG would be an adequate passer or inside scorer to fit this type of offense. (In real life a pure outside shooter may be used to 'stretch the defense' but I doubt this would work for this style of play yet in BB.
In real teams there are defensive specialists too who will man to man the best player on the opposing team whether that oponent is a small and quick guard or big forward, though this role may not be too effective in buzzerbeater unless man to man could be set up not by position but by assignment. I've gone for an all rounder sofar.
PF - Power Forward - A big man though more athletic than a center. Some are rebounding and blocking experts and have inside defense while others can shoot the ball too. Generally play inside near the basket but are much more likely to stunt outside than centers or be fast enough to play on the break.
C - Center - generally big tough and slow, they are big rebound, inside Defense and blockers and might have inside scoring, though unlikely to have any outside abilities which is how they differ to PFs. If this guy gets every missed shot by your opponents, the other 4 can concentrate on scoring is the general idea.