An archetype that any class thats an expert in one of the 4 magic related skills can take. With the GM's permission, you can circumvent doing side quests to learn rituals and be really good at casting them. You start with a +2 circumstance bonuse to pull off rituals and learn two uncommon ritual spell + 2 more at 8th, 12th, 16th, and 20th level. You then can gain feats to reduce ritual requirement: less secondary casters can cast even if one skill level lower then required, and turn a casting time of days to hours.
Ritualist starts being useful in games that go to higher levels(6lv+) and when gold is plentiful. This is a class you would only take if your solving problems with rituals, whether it is a summoner amassing servents for party jobs, That investigator which solves mysteries by interrogating the local squirrels and Grass, the party planeswalker, or making an inter-dimension guild hall manned by animated furniture.