I think Put This On gave a fairly comprehensive explanation between the types of suit, sportcoat and blazer jackets.
Some historical context is perhaps helpful. Suits emerged from formalwear and were meant generally to be worn in the context of business or other occasions calling for them. Sportcoats come from a sportswear background, in such activities like hunting and horseback riding.
The sportcoat includes details that make some functional sense. Pockets will have flaps to keep items securely inside (and rain, dirt, snow out). Often, the pockets will be at an angle instead of horizontal to make them easier to reach into while riding. The back will have a single center vent, allowing the coat to sit split over the horse’s back. Sometimes hunting and shooting jackets will have an extra collar tab and throat latch that allows the coat to be buttoned at the neck for warmth.
Of course, these are traditional details that really aren’t modern-day defining elements of a sportcoat, given that designers have borrowed and reappropriated them into suits (and vice versa).
Generally speaking, your sportcoat or blazer won’t have matching pants. They’re made of fabrics that full suits might not ordinarily be made of. They have patterns or colors that often aren’t made into full suits (think: patchword madras, tweeds, etc.). And they’re usually more casual than a suit jacket would be.