Steam is a fluid added by Thermal Foundation. The fluid currently only really exists within Steam Dynamos. It can be placed in the world, but only by spawning in source blocks as there is no Steam Bucket.
Steam from other mods, Railcraft for example, may also be used instead of this version, if registered properly.
Steam is currently not intended to be placed in the world. However, when it is placed using commands or cheats, Steam quickly flows upwards. It cannot be swum in; only walked through.