A bread knife is a long knife with a serrated (or “scalloped”) cutting edge.
What a bread knife will do is enable you to slice bread without squishing it. Its large serrations allow it to grip the crumb and the crust of bread to cut it, without having to press down on the bread to do so. Straight-edge knives have to press into bread to gain an initial purchase to start cutting.
What a bread knife won’t do is guarantee you even slices. Even with a good bread knife, some people still cut bread so crooked that their spouses are tempted to run out and buy them a mitre box.
Right-handed bread knives have serrations on the left side of the blade (to counteract the pull to the right your hand does, and help you keep the slices straight.) A left-handed bread knife, for the same reason, has the serrations on the right side.
The longer the bread knife, the truer the cut, and the more versatile it is for all shapes of bread, even round ones. Ideally, look for one 22 to 25 cm (9 to 10 inches) long. You will never regret having one too long.
You can get some bread knives which look like a fiddle bow. The blades on these, however, are easier to snap if someone uses the knife for the wrong purpose, and the knife has no real end to finish the slice off with.
You can also get electric, cordless ones.
Bread knives usually never need sharpening. Should you wish to try sharpening one at home, you need a “crock-stick” ceramic sharpening rod. You draw the blade’s serrated side down along the rod. It should take 3 to 4 passes, but test for sharpness after each pass.
Bread knives are also good for many other softer food items such as tomatoes, meatloaf fresh from the oven, etc. Many swear by them as well for cutting melons (including watermelon) and pineapple, too.
Whether or not bread knives are dishwasher safe generally depends on the handle. Some need handwashing if the handle is wood and preserving the look and long-life of the wood is important to you.
With a bread knife, use a gentle sawing motion, don’t press down. Pressing down won’t help you any, and defeats the purpose of the knife.