ANAL: At least one character is having his or her anus played with, usually penetrated, for sexual purposes.
BONDAGE: At least one character is bound, tied up, or otherwise restrained during the sex act.
FOOT: At least one character in a pose that is friendly to viewers with a foot fetish (clear shot of the sole, splayed toes, etc.,) or the scene depicts at least one character receiving attention to the foot or feet (rubbing, massaging, licking, suckling, etc.)
INCEST: Sexual relations between family members besides husband and wife. The most commonly depicted are those between siblings, or children and parents.
MAST: Masturbation. The character in question is sexually gratifying his or her own self.
MIND-CONTROL: Depicts at least one character under the influence of mind control, such as hypnotism or brainwashing.
NON-CONSENSUAL: Depitcts a sexual act involving at least one character being coerced or forced to participate against their will.
NON-ANTHRO: Depicts a character who is not at all anthropomorphic. If the non-anthro is a mythical creature (such as a dragon,) this usually means that it still has intelligence, but is quadrupedal and otherwise lacks humanoid influence in the physical sense. If the non-anthro is an actual animal (sucha s a dog,) this usually means that it is an actual, non-sapient animal, and that any sexual actions involving it would be considered bestiality.
ORAL: Depicts the use of the tongue, lips, or anything pertaining to the mouth, to sexually gratify someone.
OVIPOSITION: Depicts egg-laying in a sexual manner. POSED: The character in question is posed provocatively, usually with the breasts/genitals/etc. in full view.
PREGNANCY: Either the depiction involves a pregnancy fetish, or one of the (female) subjects depicted is pregnant.
RIMMING: Depicts one character licking, lapping, or otherwise using the tongue to stimulate the anus of another.
SOLO: The main character is by him- or herself. Instead of doing anything with other characters, the charcter is alone, either posed or masturbating.
SOUNDING: Penetration of and insertion of objects into the urethra.
TOYS: At least one character is using sex toys (dildoes, vibrators, pumps, etc.) to enhance the sexual experience.
UNBIRTHING: Depicts subjects (anywhere from small creatures to full-size subjects depending on the realism of the piece) entering or having their entire body inserted into a female subject's vagina.
UNDERAGE: At least one of the participants in the sexual activity is a minor.

F: This denotes that the character in question is female. This tag is next to a tag which denotes the species of the character as well.
F/F: Depicts two females in a sexual act (ie, female homosexuality, or lesbianism.)
H: This denotes that the character in question is either hermaphroditic (has both male and female sexual characterstics) or genderless. The boundaries and semantics between true hermaphrodites, transsexuals, etc. is wildly contested. For nothing more than the sake of sheer simplicity, the "H" label here is simply used for everything that is neither fully male nor fully female.
M: This denotes that the character in question is male. This tag is next to a tag which denotes the species of the character as well.
M/F: Depicts a male and a female in a sexual act (ie, heterosexuality.) If "(with intercourse)" is inserted after that tag, it means that vaginal intercourse (where the penis enters the vagina) is depicted.
M/M: Depicts two males in a sexual act (ie, homosexuality.)
M/M/M: Depicts three males in a sexual act (a homosexual threesome.)

ANTHRO: An anthropomorphic being, basically something that looks like a cross between a human and an animal of some sort. For example, werewolves could be considered anthro wolves, lizardmen could be considered anthro lizards, and so on.