Today the terms are pretty much interchangeable --any jacket worn with pants other than jeans or khakis is considered dressed up --although anything with two rows of buttons tends to be called "blazer" The entire story of why a garment that looks pretty much the same no matter what the name now has three different names dates back to the early 20th century and the rise of men's sportswear suits. Yes, at one time suits were considered sportswear.
The suit jacket should always come with a matching pair of pants, its what makes a suit a suit. Two piece suits were once considered casual sportswear, suitable for walking, shopping, visiting, boating golfing, in the 1920's they were commonly called "dittos" the pants and jacket matched. They were made from a wide variety of fabrics and colours. There are usually two or three buttons in a single row on the front. These became so popular that they supplanted the rigid three piece grey wool business suit and went from sportswear to dress wear. In conclusion, a suit jacket is one that come with a matching pair of pants, the jacket having anywhere from two to four buttons on the front, three the most common.
A sport coat looks like a suit jacket, and indeed it is constructed along the same methods. It is made as a stand alone garment, they aren't sold with matching pants. They are meant to co-ordinate and not match the rest of the outfit. They were also for sportswear such as golfing, hiking, boating, etc. They have a single row of buttons on the front and the buttons can number anywhere from three to six, depending on the style. With the changes in Men's fashions a sportcoat became dressier, and it became an option for dressy day wear, such as afternoon weddings, everyday business wear, or as a method of dressing up casual clothes. A modern sports coat is a slightly below hip length jacket sold without matching pants, having a minimum of three buttons in the front and three buttons on the sleeve. It's considered a bit more casual, although with the way most people dress today in jeans, t's and sweats, that's debatable.
A Blazer was once a jacket emblazoned with the crest of a private school, fraternal organization, private service club, sports club, or other group. These jackets had two rows of metal buttons on the front, denoting that they were more formal than the sportier jackets with only one row of buttons. The metal buttons would also be stamped with the crest of the group. A crest on the chest was what turned an ordinary jacket into a blazer. As fashions shifted, a blazer became any solid coloured jacket usually with two rows of buttons, usually metal. (double breasted is the correct term) and in a solid colour such as blue, grey, black or brown. It's considered dressier than a sportscoat and not as dressed up a suit jacket. Again, with the way people dress today, that's quite open to interpretation.
Tan pants and a brown jacket sounds quite nice, will you be pairing it with a simple tie also in brown? Perhaps something in a textured weave? That sounds sort of earthy and outdoors-y. If you are looking for groom's attendants, then simple dress pants with a coordinating jacket (no matter what the name) will be what you want. Don't worry too much about the name of the jacket, just go with what you think looks best. Use your eyeballs and your impeccible sense of good taste. Try to take the guys with you. If everyone decides a suit is best, then go with that.