Kissing
Kissing is pressing your lips on something, usually another person's lips. Kissing is a great activity that you should do. The lips are very sensitive (lots of nerve endings), which is why it's so pleasurable to kiss.
Kissing private parts such as the breasts and genitals is definitely sexual activity.
You can catch any ordinary communicable disease, such as the cold or the flu, through kissing. There is mononucleosis?, sometimes called "the kissing disease", which you can catch by kissing.
You cannot catch most STIs (sexually transmitted infections) through kissing, so kissing is pretty safe. But you can catch oral herpes?, which gives you cold sores and is highly infectious. Please see the article on oral herpes (when I write it) on how to avoid spreading it. In the mean time, at least don't kiss during breakouts (or even when you feel a funny tingling feeling in your lips that comes before breakouts), and not immediately after them, either. Or kiss through a latex strip?. Or kiss someone who already has oral herpes.
I have heard that open-mouthed kissing means you want to have sex, but I don't know that for sure, you may want to avoid open-mouthed kissing if you want to avoid having sex. Open-mouthed kissing where you put your tongues in each others' mouths is called French kissing, or just tongue kissing. Which is a great, enjoyable activity that people should do. If you do open-mouthed or tongue kissing, try to prevent your saliva from leaking into the other person's mouth. It's just polite.