The least formal of the trio, the sport coat was traditionally used for hunting and other sporting activities or events. Sport coats are generally sturdier than the others, and oftentimes feature more patterns or other rustic designs.

Distinguishing characteristics include: 

• Ability to layer, less structured and fitted 
• Wider array of colors and patterns 
• Elbow patches and pleats 
• Flannel, tweed, houndstooth, herringbone

Today, sport coats can range from the quite casual to fairly formal, and there isn't a one size fits all style. Typically seen as more of a casual addition to a button down and pants/jeans, patterned styles and colors are welcome.

Blazers are more formal than sport coats and traditionally made from a blue wool fabric with distinct silver or gold buttons. However, as years passed, blazers have delved further from tradition and come in more variety. 

Distinguishing characteristics include: 

• More shoulder structure and fitted 
• Traditionally gold or silver buttons 
• Neutral, solid colors

Today, with the fall from favor of gold buttons and the increase in style variety, blazers are almost indistinguishable from sport coats. Feel free to use these terms more or less interchangeably.

The more formal of the trio, suit jackets are generally made from finer wool (with a few exceptions for summer suit jackets made from cotton or linen). Suit jackets are always sold with matching pants, and occasionally a matching waistcoat as well.

Distinguishing characteristics include: 

• Always sold with matching pants 
• Solid construction and tapered fit
• Typically the most expensive

With the differentiation between sport coats, blazers and suit jackets fading away, suit jackets can often be worn as blazers or even sport coats, especially neutral tones. When shopping for your next suit, keep in mind how you might wear the jacket as a sport coat as well.