Introduced in Pokémon X and Y, this beloved feature can be accessed via Festival Plaza's "Trade" menu, while connected to the Internet. How it works is that you select a Pokemon to trade away and the game randomly assigns you a trade partner from anywhere in the world who happens to be using Wonder Trade at the same time.
Both of you have absolutelyno idea what Pokémon you're getting until the trade actually occurs. There is no feedback involved in the process, so don't expect any negotiations or anything like that. Wonder Trade is great fun because you never know what Pokémon you're going to get and it's also incredibly straightforward to use.
Good in Theory?
Wonder Trade is not without its downsides though. Because of the anonymity, there is no benefit to trading away actually good Pokémon, so chances are you'll be receiving junk most of the time. Think of it like the GTS, where people just want to get rid of their Pikipek and Yungoos.
However, as they say, somebody's junk can be somebody else's treasure. Perhaps this junk is a Pokémon you don't yet own. Or maybe it's a reject Pokémon from somebody's breeding project that you can incorporate into your own breeding project. Or if it's literally junk, hopefully you didn't send anything too valuable...
Even if you do miraculously get something good, that's when your alarm bells should be ringing. While there are some very kind people in the world, the chances of you getting a shiny Pokémon or Level 100 Pokemon with 6 perfect IVs with no strings attached is astronomically slim.
Should you receive such a Pokémon, you can confidently assume that it's cloned (because you don't lose anything that you clone) or, worse, outright hacked. Now, there are no adverse effects of owning cloned or hacked Pokémon, especially if they're for your personal collection, but it's something to keep in mind.
If you're intending to participate in official competitions, we strongly advise not using Pokémon obtained from Wonder Trade. Instead, try to use your own Pokémon or Pokémon traded from people you actually know. Despite what the company says, breeding Pokémon from unknown origins is fine so long as it doesn't have impossible traits.