Touching, holding & kissing - these are what they sound like, and can be used as part of foreplay with a partner.
risks - these are all low risk activities, although if you have cuts or sores either on your body or in your mouth the risk of passing on an infection is higher. There are some STIs which could be passed on with genital contact - if in doubt, wear a condom.
Masturbation - is stimulating your own clitoris (female) or penis (male). This can be continued until you have an orgasm, and until you ejaculate semen if you are a man. Masturbation is a good way to explore your sexuality and discover what you like and don't like, so you can let any future partners know what to do and what to steer clear off.
risks - as you're going solo, masturbation is no risk.
Mutual masturbation -is masturbating a partner, or the other way round. Can be used as foreplay before penetration, or continued to orgasm.
risks - this is low risk, as long as any fluids (such as vaginal secretions or semen) don't enter any cuts or sores you may have.
Oral sex - is either stimulating the penis with your mouth and tongue, or doing the same to the clitoris and vulva. Can be continued to orgasm or used as foreplay.
risks - oral sex is officially a low risk activity. However, the level of risk may be increased if you have sores or ulcers in your mouth - if in doubt, use a condom (flavoured if you like) to stop any semen or precum entering your mouth, or use an oral shield if you're performing oral sex on a woman. For the same reason, don't brush your teeth just before having oral sex as you can quite easily make small cuts in your gum. If you've got bad breath, chew some gum.
Vaginal sex - is inserting the penis into the vagina. Can be done in a huge number of positions and variations. This is what most people call 'sex', although it's worth noting that many women will not achieve orgasm purely through vaginal sex as the clitoris is not directly stimulated.
risks - the most obvious risk is unplanned pregnancy if you don't use a barrier method or any form of contraception, and many women may take the Pill to prevent this. However, the other major risk of unprotected vaginal sex is catching a sexual infection, including the easy transmission of HIV. The Pill will not protect against this - only a barrier method such as a condom will prevent unplanned pregnancy and offer protection against sexual infections.
Anal sex - is inserting the penis into the anus. As the prostate, or 'male G-spot' can be stimulated in anal sex, the experience can be very pleasurable for men. As there is no natural lubricant, you'll need some of your own. Make sure you use plenty and take it slowly, allowing the sphincter to relax. If it hurts, stop and give it time to relax, or maybe add some more lube.
risks - the delicate lining of the anus can be damaged by anal sex, particularly if there is insufficient lubricant, leaving cuts for sexual infections to enter. As such many sexual infections can be transmitted by anal sex, including HIV.
Fingering - is using the fingers to either stimulate the clitoris and labia in women, or to stimulate the anus, possibly including entering the anus and stimulating the prostate for men.
risks - this is low risk, but the risk of infection can be greater if the delicate skin inside the vagina or anus is damaged. To stop this happening, make sure your nails are trimmed, and don't force anything.
Rimming - is licking and penetrating the anus. Enjoyable for men as the prostate is stimulated.
risks - there is a slight risk of infection so use a barrier such as an oral shield if you like.
Fisting - penetrating the anus or vagina with your hand, and possibly some of your arm!
risks - there is a risk of seriously damaging the tissue inside the vagina or anus, and it's possible to rupture the anus. Use plenty of lube and a latex glove.