Temperament refers to the typical behavior that a fish exhibits in a community with other fish.
Peaceful - This means that this particular fish is most often very peaceful, and would make a great community fish. These fish usually will very rarely attack other fish, unless during special times, such as during breeding. Peaceful fish won't usually eat other fish unless if they are very small fry, which would only be most likely an accident.
Semi-Aggressive - This means that it is not entirely peaceful, but not quite aggressive either. These fish are usually more likely to be a small conflict to other fish in the community by doing this such as, fin-nipping, or eating fish small enough to fit in their mouths, or are somewhat territorial. Though, these fish still make good community fish members, if you pick good tankmates, and monitor some actions that are somewhat suspicious.
Aggressive - This means it will be very territorial, and will attack many members of the community, especially smaller ones. These fish also are known to eat fish that are smaller. These fish don't make good community members, but will tolerate bigger fish, or fish that cannot fit in their mouths.
Species - This means it is so aggressive to other fish, except for those of the same species, that they are best kept by themselves in a species tank.
Solitary - This means that this particular fish is best kept in a solitary tank by itself. This is most likely due to the fact that they will virtually attack, or even eat any other fish in the tank no matter the size, so it would be best to keep this fish is best kept by itself in a fish tank, with no other fish.