The one real difference
A stallion is an entire male horse: he has been left uncastrated, keeps his full male hormones, and can sire foals. A gelding is a male horse that has been castrated — the operation, usually done in the first couple of years of life, removes the testicles and with them the great majority of breeding drive and hormone-fuelled behaviour. You may also meet the word colt for a young entire male and rig for a horse incompletely castrated, but stallion and gelding are the two everyday categories. Nothing else about the horse's breed, size, or ability changes; the difference is reproductive status and everything that flows from managing it.
Build and behaviour
Stallions typically develop heavier crests, thicker necks and jowls, and more muscle than geldings of the same breed, because testosterone shapes the body as they mature. Behaviour is the bigger gap. A stallion is alert to mares, may call, posture, and challenge other males, and can be single-minded to the point of danger with an inattentive handler. A gelding's attention stays on his work and his rider; he is generally even-tempered across seasons and company. This is why geldings are the standard riding horse for lessons, children, and leisure owners, and why many sellers geld a colt before sale to widen his market.
Management: where a stallion costs you effort
A stallion needs his own turnout or carefully chosen company, fencing that is high, strong, and respected, and stabling arranged so he is not nose-to-nose with mares or rival males. Many livery yards will not accept stallions at all, and shows and fun rides often exclude them or require a stallion disc on the bridle and an experienced handler. Handling one is a skilled job: routines must be consistent, personal space enforced kindly but absolutely, and breeding duties, if he has them, kept clearly separate from ridden work. None of this makes a stallion a bad horse — well-managed stallions compete at the highest level — but it makes him the wrong horse for most setups.
Gelding: timing and what changes
Most colts destined for ordinary riding life are gelded young, often before two years old, once both testicles have descended and before stallion habits set in. The operation is routine but not trivial: it is surgery, with aftercare, a risk of swelling or infection, and a recovery period of gentle movement. Behaviour changes over the following weeks as hormone levels fall, though a horse gelded late, after years of covering mares, may keep some stallion-like habits for life. Gelding also ends any breeding value permanently, which is why genuinely well-bred colts with stud potential are left entire until their quality is proven.
Deciding which you are looking at
If you are buying, the practical question is fit: an experienced home with facilities can enjoy a stallion's presence and quality, while almost everyone else is better served by a gelding's straightforwardness. Either way, the horse's status belongs in writing in his paperwork, not guessed from his behaviour at the viewing. When a horse is sold on through KinsLe, his Transferrable Profiles record states plainly whether he is a gelding or an entire stallion, alongside his history and routines, so the buyer arranges the right fencing and turnout before he arrives rather than after.