When you receive so much anal sex, that your whole ass gains a complexion of a beautiful tan. But this is not from the sun, but rather a mix of shit, blood, cum and lube. This is usually applied to gay men, and is complimentary.
A bed/booth you lay/stand in. It has ultraviolet bulbs that emit UV very much similar to the sun. It will give you a tan very much like the sun does. There is a possibility skin cancer can be acquired from using one of these devices.
Tanning injections refer to the injection of one of the melanotan peptides (melanotan-1 and melanotan II) to develop pigmentation in one's skin thus allowing individuals who ordinarily cannot tan or have difficulty doing so (fitzpatrick skin type I and type II, ie: redheads and gingers) to be able to do so. See: Melanotan.org