Flush trim bits have bearings, some straight bits do not. The cutters on a straight bit are straight up and down, while flush trim bits usually have a slight angle to them. This angle gives a better cut because the cutter is slicing the wood off instead of hacking it off.
For most work they could be interchangable, but the better cut is from the flush trim bit.
The flush trim bit cannot be used as a groove cutting bit, for making dados for example, because of the bottom mount bearing. A straight cutting bit (preferrably one designed for plunging) is best for groove cuts like that.
Some straight cutters have pilot bearings on the shank, above the cutter, and are called pattern bits. They can usually be used as dado bits, but are usually smaller diameter cutters with 1/4 in shanks, and I really don't like 1/4 inch shanks unless I cannot find a 1/2 in shank.
A small set that mixes the best of both worlds is the GRIZZLY.com H5567 Straight Flush Trim 3 pc. Set, 1/2" Shank. This set has a standard 1/2 in flush trim bit (bottom brg.) and a 3/4 in top bearing pattern bit, and a 1-1/8 in top bearing pattern bit. For $20 bucks, these bits were a steal (the 1-1/8 bit did need a little honing but nothing unreasonable). While not a plunge cutting straight cutter, the 3/4 in pattern bit would work well for cutting dados, in multiple passes.)
Hopefully this helps a little. The only other thing I could think of is to go to the Whiteside, Amanna, WoodlineUSA.com, or PriceCutter.com and look at the pictures of the various bit configurations. You will need more than one type. A straight bit with template guides can do a lot of the work of a pattern bit, but really can't flush trim. A flush trim bit can cut a lot of straight edges, but cannot plunge cut because of the bottom bearing.
Any straight bit can be used kind of like an edge jointer, if you have the outfeed fence of your router table shimmed out a 16th or so.
One other thing to keep in mind is the fact that a spiral bit IS a straight bit. To understand the difference in the cuts try pushing a knife blade down through a tomato, then try using a side to side slicing motion. This slicing action is what happens as the bit spins, ergo a smoother cut.
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