Most modern smartphones can view video encoded in MP4 (MPEG4) and 3GP (H.263) formats without conversion.
Downloadable mobile porn is that which can be made portable and taken with you, like downloadable adult movies or short clips of porn movies.
Streaming porn phone services are those which require an internet connection, like streaming video or video on demand, and usually work best on devices with 3G or WiFi.
If you know the type of content or niche you want to find, choose it from the list below to be taken to a page of more specific options.