What is the difference between a bike path (or mixed use path) and a sidewalk?
You're allowed to ride on a bike path, you're generally not allowed to ride on a sidewalk. The visible difference is the signage on the bike path saying bicycles are allowed there.
A sidewalk is normally the default label for any non-vehicle path next to a road. They're almost always restricted to pedestrians and mobility vehicles (prams, wheelchairs), usually with wheeled toys also allowed. The detail of how those are defined varies, but in general it's "things that move at pedestrian speeds".
Mixed use or shared paths are distinguished by their signage. Somewhere near the start and end will be signs saying "bicycles allowed" or "horses, bicycles and motorbikes allowed" or something similar. Often in graphic form rather than text. The local laws will define exactly where those paths start and end, but the people building them often don't know or care what the law says, leading to situations where cyclists have to dismount, walk their bike one or two metres, then they can ride on the shared path. Obviously no-one does that, but occasionally someone gets ticketed for it (or presumably in the USA, shot).
Specifically in Maryland:
Shared-Use Path A bikeway outside the traveled way and physically separated from motorized vehicular traffic by an open space or barrier and either within the highway rightof-way or within an independent alignment. Shared-use paths are also used by pedestrians (including skaters, users of manual and motorized wheelchairs, and joggers) and other authorized motorized and nonmotorized users. (Maryland MUTCD, 2006)
Sidewalk That portion of a right-of-way designed for preferential or exclusive use by pedestrians.
This official pdf seems to have collected all the bicycle-related parts of Maryland code and there's a summary on an official website. You can only ride on a sidewalk where specifically permitted by local rules, and interestingly wheeled toys, unicycles, and mobility scooters also need special permission to use sidewalks. They don't mention prams, so I suspect "wheeled thing that you push or pull" counts as a pedestrian.