When you set some animation keyframes to an object Blender stores your keyframes in an "Action". The dopesheet shows all actions of the scene. You can set the dopesheet window to "Action editor", select an action and press the "Push down" button to create an "NLA Action strip", which contains the same informations of the action, but can be manipulated with all the NLA options.
So, an "Action strip" is a Blender datablock, that refers to a certain object in a certain scene of a certain file: it's true that you can apply it to a different object, but when you have a rig maden of multiple bones it's very difficult to find an action strip that will work with a different rig, and this is the reason why no one share action strips on the internet: they would be pretty unuseful.
Bvh files are actions derived from a live action: they work well when you apply them to a rig identical to the one used to capture the live performance, but if you want to apply them to a different rig, you will have to perform a complex operation called "retargeting": there are various addon that can help you, but it's not a beginner task: if you want to follow this line, search for "Bvh retargeting" and "Bvh workflows".