Summary:
Beach shorts are a special kind of shorts that are designed to get wet
Beach shorts are a special kind of shorts that are designed to get wet. They are not swimming shorts, but they are not normal shorts either - they are a special kind of shorts designed to look like normal shorts, but at the same time designed to get wet with seawater and dry quickly with no ill effects.
They are generally made of materials like lycra, flannel or nylon, which are chosen to be quick-drying but not too warm. They're a good alternative to trunks, and are often considered both more fashionable, especially among young men, as well as more modest in that they don't reveal so much. However, some shorts still come with a trunks-like structure on the inside, to offer the support of trunks while still giving the look of shorts.
Although most people only wear beach shorts when they're going to the beach, it is common for people who live in beach towns to wear beach clothes all the time, in case they happen to feel like going to the beach one day. This is quite easy to spot when you walk around one of these towns, and can make you wonder whether everyone is on their way to the beach!
Beach shorts aren't just for standing around in the sea, though - they're also just fine for wearing on the beach itself, and surfers often use them as well. Surfers have to make sure to use good shorts, however, as cheap ones are prone to coming off when you hit the water hard.
One word, though: Hawaii-style beach shorts aren't fashionable, and hopefully never will be again, so just don't. Stay away from the big, unsightly patterns in your beach shorts, and go for plain colours instead. Trust me, you won't regret it. It's also worth mentioning that you should probably go for dark colours rather than light ones, as light colours can become see through when they get very wet, and chlorine from swimming pools you use them in can make colours fade.