All characters are automatically familiar with any weapon that is related to a weapon they are proficient in. Weapons are considered to be related if they are part of the same weapon group by type, or cultural weapon group. Characters are only familiar with the weapons in those cultural weapon groups they have affinity with. (Characters automatically gain affinity in a new cultural weapon group every 3 levels)

For example, a character is who proficient in the use of the Long bow is automatically familiar with all bows (weapon group by type). That same character also happens to have affinity with the cultural weapon group from Cormyr. Because that group includes the Long bow, he is also automatically familiar with all the other weapons in that group (Long sword, Heavy Lance, Crow’s Beak, Halberd, and Flail).

Unless he also has affinity in different cultural weapon groups, that familiarity does not extend to every other cultural weapon group that happens to include the long bow in its list (Cormanthor, for example).

At level 3 he gains affinity with a new cultural weapon group, and he chooses Cormanthor. That familiarity will now extend to all the weapons in that group as well (Short bow, Composite long bow, Longsword, Shortsword, and Dagger). There will inevitably be a lot of overlap, and that’s by design.

Familiarity is not as good as proficiency, but it beats not knowing anything about a weapon at all. Characters only suffer one-half the normal non-proficiency penalty when attacking with weapons they are familiar with. They may attempt any normal attack maneuvers possible (the familiarity penalty still applies, of course), and suffer no initiative or rate of fire penalties.
Familiarity does not allow the user to make use of any special weapon attack modes that require proficiency in the weapon.