Personality Types are basically simple ways of describing a character's personality. While it doesn't list every aspect of the individual's personality, it can give a basic idea of how the person acts.
Knights are individuals who feel the need to help others. Whether by protecting them from bullies or protecting them from a more...serious...threat, a knight will not lightly leave a person in need to their fate.
Paladins are a step up (or, depending on how you look at it, down) from Knights. Paladins are individuals who feel a deep, psychological need to help others, actively seeking those in need of aid, for the sole intention of aiding them.
Pacifists absolutely abhor fighting of any form, for any reason. For this reason, they will not participate in a battle, regardless of the reason for doing so. In this current galaxy, however, Pacifists are growing rare, unless it is a racial trait of them.
Semi-Pacifists are those individuals that, like Pacifists, do not like fighting, but, if pressed into a battle, unlike a pacifist, the Semi-Pacifist will fight, if only to go as far as protecting themselves or their companions.
It is not uncommon for someone with the Semi-Pacifist Personality Type to eventually move to another personality type, usually to either a 'Knight', shedding the Pacifistic hesitation to fighting; or to a full-fledged Pacifist, swearing off fighting all together.