If you have ever wanted to wear your suit jacket as a blazer, with a pair of khakis or jeans, then this is a question that surely you have asked. For the majority the correct answer is just plain and simply confusing. Many have chosen to interchange the three and basically called anything with a pattern, but worn with non matching pants, a sport coat and a solid colored version, again worn with non matching pants, a blazer. As prevalent as this practice is, it is not the definition of the three.
This is some thing to be worn with its matching trousers. For the most part the silhouette of the suit jacket flows down to its trousers so matching the jacket with other pants produces a weird look. If you have ever tried this and felt like the pants didn't look right-as is you didn't dress yourself-its not in your head, this is exactly what it looks like. A suit jacket will be constructed, it will have canvas interlining and shoulder padding. It is made to look tailored and dressy. The details will also be more on the formal side, with piped pockets (with or with out flaps) constructed lapels, lining and bone or corszo buttons.
A sport coat is something that is worn for a more casual setting. At its purest form it was something worn to go hunting so this is why you usually see sport coats in heavier more patterned fabrics (this was the English early version of camouflage) A sport coat is made with a more relaxed look, usually it will have either a half canvas interlining, or no canvas at all. It will have extremely light padding to no padding on the shoulder and it will have a more functional fit. In today's world of custom tailors and tech fabrics, heavy tweeds and a big baggy fit are no longer necessary, unless you are actually going hunting. If you are only wearing the sport coat as a jacket, look for a more tailored fit similar to the suit jacket, its deconstructed look will give it a nice casual feel.
If boats are your thing then the blazer is a piece of clothing that you are familiar with. Born from the British naval uniforms, a blazer is typically blue- though, much like the modern sport coat, where its a fashion statement rather than a functional piece of clothing- it doesn't always have to be the case. It will have a constructed feel, just like the suit jacket, full canvas interlining, shoulder pads etc. The main difference is that it will be made with details that are more casual, like patch pockets, contrasting buttons (gold or silver).
With all the options available, from off the rack to custom suits finding a brand or a tailor that is consistent with the design and shapes of his cuts can help expand the versatility of your wardrobe. This will more freely allow you to mix and match your clothing.
Chris Vance is the senior trends and fashion spotter for Requisite clothing, a custom clothing design house specializing in custom suits, custom shirts, custom tuxedos and sports wear for both men and women.
Source : http://ezinearticles.com/?The-Difference-Between-a-Suit-Jacket,-a-Sport-Coat-and-a-Blazer&id=2503631