What is the difference between a grand piano and a baby grand?
'Baby grand' is a marketing term rather than one of technical specificity. Grand pianos 5'8'' and shorter are frequently called baby grands, although there are endless exceptions. So, the difference between a grand piano and a baby grand is arbitrary and not a particularly important distinction to make, unless of course you are trying to sell a grand piano to someone with limited space, and then the descriptive language used can have a big effect on perception.
Based on my observations of how people use the term 'baby grand', it seems that for many it conveys a certain social cache and suggestion of high-quality craftsmanship. Why this is so is not exactly clear to me. In many cases, a full-sized upright will have a fuller sound than a 'baby grand' because of its longer string lengths in the mid- and lower-registers. And 'baby grands' span the gamut from well crafted to poorly rendered.