Depends on what you mean when you use the term "monitor." Many are large, full range speakers.
Are you refering to the difference between bookshelf (those that use stands) and floorstanders which generally sit on the floor? This sounds like what you are really wanting to know.
Monitor and fullrange are vague. Not all bookshelves are monitors and not all floorstanders are full range or even close to it. Even what is full range is debatable and opinions will vary.
Generally monitors are smaller speakers which are designed to be used with a stand. Fullrange speakers are generally much larger and sit on the floor. However most floorstanders are not full range and there are some monitors which can surpass some floorstanders in the bass department.
Yes you can use either floorstander or monitors for you HT setup. Both are commonly used. Chances are if your going for HT you'll probably want a subwoofer so either should be just fine.
Hope this made some sense???