Help: Ratings
All posts on Rule34 have one of three types of ratings: Safe, Questionable, and Explicit. Questionable is the default rating if you do not choose to specify one. Please take note: occasionally explicit images will be marked safe, and vice versa. You should not depend completely on rating filters unless you can tolerate the occasional wrongly rated image. If this happens to you, fix it so that other users don't have that happen to them.
Explicit
Any image where the vagina or penis are exposed and easily visible. This includes depictions of sex, masturbation, or any sort of penetration.
Safe
Safe posts are images that you would not feel guilty looking at openly in public. Pictures of nudes, exposed nipples or pubic hair, cameltoe, or any sort of sexually suggestive pose are NOT safe and belong in questionable. Swimsuits and lingerie are borderline cases; some are safe, some are questionable.
Questionable
Basically anything that isn't safe or explicit. This is the great middle area, and since it includes unrated posts, you shouldn't really expect anything one way or the other when browsing questionable posts.
Search
You can filter search results by querying for rating:safe, rating:questionable, or rating:explicit. You can also combine them with other tags and they work as expected.
If you want to remove a rating from your search results, use -rating:safe, -rating:questionable, and -rating:explicit.