The short version: Originally it’s just paper mail. Your bills, letters and everything you get in your mailbox. The snail-part refers to how slow it travels (compared to email for example). Source: wikipedia.org
Nowadays there’s a great trend with the same name. It’s basically the same, except you’re invited to add fun little goodies. These can be handmade, thrifted, bought or vintage: Whatever you think the receiver will like. (please note! there are various opinions about what snailmail is; I am not an expert, this is just my point of view)
Snailmail should be fun. Take your time to create something and enjoy what you receive. It’s not only about the goodies, the letter is just as important. It’s fun to get a little peek into someone else’s life, so share yours too!
A snailmail package usually consists of: a decorated envelope, little goodies and a (preferably) handwritten letter.