Typically, this will require the user to click a button confirming that they are over 18, that it is not illegal to receive pornographic material in the area where they live and that such material does not offend against prevailing community standards. Most seem also to support the technological 'locks' which parents can impose on the Web on their machines. Most sites offer free samples, a kind of loss-leader to entice the user into the site. Actually entering the site requires a fee, typically to be paid by credit card or telephone bill. In the images of heterosexual acts, anal intercourse and fellatio seem to be quite common.
Some sites also offer 'movie' clips, which seem often to show fellatio and cunnilingus.
Straightforward vaginal intercourse seems to be fairly uncommon, which concurs with the findings of the Carnegie-Mellon survey of newsgroups. The verbal descriptions of what is available to the paying site visitor do sometimes appear to be designed to titillate paedophilic interest by stressing the youth of the woman featured, typically 'teens', but I have seen little evidence that someone who pays the fee will find anything 'out of the ordinary', simply because it would be illegal - which is in fact conceded in the statements on sites claiming to show 'barely legal teens' etc. that none of their models is under 18. Newsgroups seem to be another matter. Many of the postings also stress the youth of the featured model and, in some cases, the illegality of the pictures. In one case, to my shock, I discovered a photo of what appeared to be a naked girl aged around ten, blindfolded and tied to a table.