There really is no difference. Both are "revolving spool" type reels. Generally, those that can be cast one handed are considered "baitcasters" which is the type that a lot of bass & inshore folks use and are usually cast with one hand rods. However, surf casters use them as well, but they may be a larger size reel and often cast with 2 hands because the rods are longer & have longer handles. Not something that most folks would try to hold & cast with one hand & most wouldn't fit on a rod that is intended to be cast with one hand. I say generally too, because there are so many rod & reel combinations now, which was not the case at one time. For example, an ABU 6500/6600 size reel can fit on a single hand rod, or a longer surf rod, and there are other similar size reels. That same model reel can have a level wind or it may not. However, most "conventional" reels used for casting will not have level winds. That feature is great, but can limit distance. So, conventional are usually considered the non-level winds.
The bigger reels, which are used primarily for trolling are not usually referred as baitcasters, simply because no one casts them, they're too darn big & heavy & the rods used with them are not casting rods, they're trolling rods.
The main difference between conventional & baitcaster is the person using it & their definition of what each entails. Not always cut & dry.