First of all, congratulations on taking the responsibility to find out for yourself how you look and what your own anatomy is like. It is a great first step to a healthy sexual experience.

The pink is most likely your hymen. What is behind it is your vagina. The vagina is normally like a fireman's hose that folds flat onto itself. if you look at it from outside it will look like a slit going from left to right between your legs. It is actually in the form of an H with the two upright legs on the left and right and the horizontal bar between them. Only when you are sexually aroused will it open up and become the 'dark cave' you expected.

Pre-puberty your hymen is thin, translucent (with some veins visible) and not very elastic. Some time in puberty (for some girls in the beginning, for some at the end) the estrogen in your body will make the hymen thicker and form more of a ring around the outside of the vaginal opening. This ring is very elastic and can be a full 360 degrees ring or just around part of the vaginal opening. I have recently heard somebody describe the ring as the same as the StarGate from Startgate Atlantis.

The hymen comes in different shapes (imperforate, annular, crescent, fimbriated, septate, cribriform). You can google 'hymen shapes' and will find lots of useful diagrams on the different shape hymens.

It can cover the whole vaginal entrance or just a section of it. It should have a hole in it for menstrual blood to exit. It can have a bug uneven hole, many small holes or 2 holes with a fleshy 'bridge' between the holes.

You cannot do anything for your hymen, just like you cannot do anything about your ears or lips or fingers. Through your normal life it can stretch or tear a bit. Anything that will stretch it and shake it violently (like gymnastics, dancing, karate, judo, horseback riding, skiing, waterparks, mountain bicking and skate boarding) can cause it to tear or stretch spontaniously.

Using tampons, sex toys into the vaginal opening, fingering, or being examined with a speculum can also tear it.

For the most part it is the way it is after puberty. Some doctors will prescribe an estrogen paste/cream to rub on it to speed up the thickening and elasticizing of the hymen. Other doctors will make a small incision in it under local anesthesia.

I do not think it is something to be scared of. At some stage you will become so sexually aroused and turned on that it will not matter to you.

Don't be pressured into intercourse just because you are worried about your boyfriend. He probably hangs out with you because you are such a nice and interesting girl, not because he wants sex with you. What he is doing is perfectly OK and a form of outercourse (the opposite of intercourse), and will relieve the sexual pressure and frustration he is feeling. Also remember to always be safe when you play with him and make him wear a condom. Even though you are not having intercourse, you should still keep his semen away from your vaginal entrance.