This is the list of activities: https://github.com/ValveSoftware/halflife/blob/master/dlls/activity.h
The game looks up which animation has the activity set for it. Check the QC file for which animations have which activities.
As far as movement goes, each animation defines how far it moves. The root bone is used to determine where the animation starts and ends, end - start (position in last frame - position in first frame) determines how far movement goes, the rotation defined for the animation affects which direction movement will go in.
Movement speed is determined by movement distance * (framerate / number of frames - 1). The fewer frames and the higher the framerate, the faster movement will be.
If there are no frames then movement speed is always 0.
None of the models in the SDK specify rotation, so don't be surprised if it doesn't work properly.
The function used to extract movement speed can be found here: https://github.com/ValveSoftware/halflife/blob/5d761709a31ce1e71488f2668321de05f791b405/dlls/animation.cpp#L226
pflGroundSpeed is movement speed, pflFrameRate is the framerate for the animation, which is independent of the entity framerate. The final framerate is pflFrameRate * entity framerate, so movement speed will be affected by that as well.
The actual movement is performed here: https://github.com/ValveSoftware/halflife/blob/5d761709a31ce1e71488f2668321de05f791b405/dlls/monsters.cpp#L1980
Some entities may override this so check their MoveExecute method. If you wanted to, you could add a keyvalue that overrides the animation movement speed, or that multiplies it to speed up/slow down the entity.