@ Meow Mix
There are a number of reasons why Islam prohibits tattoos, Cocolia has mentioned a few of them.
I will explain this matter a bit more especially the parts that Cocolia hasn't mentioned.
Firstly if one has/had tattoos while they were not a Muslim and latter become one, they have all their sins forgiven. However if tattoos are in question then after or just before accepting Islam one needs to remove their tattoos due to the matter of cleanliness.
Tattoos are forbidden for two main reasons, alteration of the body (this part has been explained by Cocolia) and prevention of water flowing over/through ones skin.
One of the conditions of ablution and ritual baths in Islam is that water must flow through/over ones skin. A tattoo does not allow water to flow over the skin (i.e water doesn't touch that part of the skin over which the tattoo is), it is the same as having a piece of shrink wrap over ones skin or as any other kind of paint.
Second, it has been asked 'why specifically women'. The thing with hadith is that just like the Qur'an, at times a particular hadith is directed at a particular group or individual due to that group or individual having taken part in something prohibited by Islam.
If there is no hadith that speaks of men being prohibited from getting tattoos the most likely reason for that is because at the time of the Prophet it was not the custom of men to get tattoos but instead was the custom of women only. So to deter them from this practice a ruling directed at women was issued which of course applies to the men of latter generations were they to take part in this prohibition.
And as for the limits to alteration of the body, if any part of ones body doesn't look natural then it is permissible to alter such a part into looking natural. But if any part of ones body looks natural then it is not permissible to change it unless it is due to medical concerns. Example:
We all have appendixes and we know they can be painful, but unless you are experiencing any pain from it there is no reason why you would want it to be taken out. There are people who live and die and have no problems with their appendix.
As for circumcision, it was the custom of Prophet Abraham, as mentioned by Cocolia. The reason behind circumcision is cleanliness. STD's are far more likely to be transmitted through an uncircumcised man than through a circumcised man.
I hope that clears a few more things.