There are many different things you can do during sex, and what sex is can be different for everyone. Very generally, you can think of sex as anything that makes you feel "turned on" and feel good in an intimate way. Sex can be masturbation, where you touch your own genitals, either by yourself or with a partner. Sex can be oral, vaginal, or anal. Oral sex uses one's mouth to stimulate a partner's genitals. Vaginal and anal sex are both penetrative, meaning that vaginal sex is when a penis or sex-toy goes inside a vagina, and anal sex is when a penis or sex-toy goes inside an anus.
To some, sex can also be considered kissing, rubbing or grinding bodies together, as well as sending sexual texts (sexting). People can have different definitions of sex and view sex very differently. One of the most important things during sex is to communicate with your partner or advocate for yourself during sex to make sure what's happening is comfortable and safe for all parties involved.