ESRB have not long updated the full ratings summary on their website, giving some tidbits of information about just why GTA V received it's mature rating.
Now, these might be considered slightly spoilery as they do reveal some minor, but interesting, elements of gameplay, however, if you're concerned just check out GTA IV's rating summary to get an idea of what may be detailed.
- Players also have the ability to shoot non-adversary civilians, though this may negatively affect players' progress as a penalty system triggers a broad police search.
- Blood-splatter effects occur frequently, and the game contains rare depictions of dismemberment.
- In one sequence, players are directed to use various instruments and means to extract information from a character.
- The game includes depictions of sexual material/activity: implied fellatio and masturbation; various sex acts that the player's character procures from a prostitute while no nudity is depicted in these sequences, various sexual moaning sounds can be heard. Nudity is present, however, primarily in two settings: a topless lap dance in a strip club and a location that includes male cult members with exposed genitalia in a non-sexual context.
- TV programs and radio ads contain instances of mature humor: myriad sex jokes; depictions of raw sewage and feces on a worker's body; a brief instance of necrophilia
- Some sequences within the larger game allow players to use narcotics (e.g., smoking from a bong, lighting a marijuana joint); cocaine use is also depicted. Players' character can, at various times, consume alcohol and drive while under the influence.
- The words f**k, c**t and n***er can be heard in the dialogue (we'll let your imagination decide what the words are.)