A couple of terms you will come across are “top hung” and “bottom rolling”. They refer to how the weight of the doors will be distributed when the doors are folding/sliding.
All bi-fold doors have tracks in the top and bottom of the frame that the doors slide along in a train fashion. With top hung doors, the doors are “hanging” from the top track, (so all the weight is at the top) and with bottom rolling doors they roll along the bottom track (so all the weight is at the bottom). Both systems will have a guide that runs along the other track.
Be aware that some companies will tell you that one is better than the other. These companies are likely to only sell one of the systems, and therefore say that the one they sell is the better one. In reality, one is no better than the other.
One thing that is worth noting is that with top hung doors, you will have to ensure that the lintel above where the doors are to be fitted is strong enough to take both the weight of the doors and the weight of whatever is above the doors (wall, roof etc). With bottom rolling doors all the weight is at the bottom, so this doesn’t apply.