Missingno is a glitch Pokemon that only exists in the original Red, Blue and Yellow games. It can be encountered through the use of the Old Man glitch, which can be performed by watching the old man in Viridian City catch a Weedle.
Basically, what this does is it puts your trainer's name in the wild Pokemon encounter coding for a narrow strip of land to the right of Cinnabar Island. This causes a glitch as the programmers left this area blank and so the game send a whole ton of glitchy nonsense; and what you can find there depends on what characters are in certain parts of your name. Sometimes you can find Pokemon over 100, and sometime you can find Pokemon that you could only find in the Safari Zone normally; but this glitch is most well known for brining up Missingno.
Missingno is a Bird/Normal type, which might mean that Flying was originally Bird type early in the game's development but it was later replaced with Flying. Missingno also has different "forms"; the normal glitchy "L" shape being the one that most people recognise. If also appears like the ghost in Lavender Town, a Kabutops Fossil sprite, and a bunch of glitchy yellow colour, and which "form" you encounter depends on your name also.
One thing commonly misunderstood is that Missingno wrecks your game and glitches it up forever. The truth is that Missingno is actually surprisingly stable and the only thing it screws up for good are your Hall of Fame entries, but the game isn't unplayable without those. It also scrambles the sprites of many Pokemon, though this can be fixed bye looking at the summary page of a legal Pokemon. It rarely causes the game to freeze (and if it did it would be likely caused by a separate glitch considering how buggy the original games were).
Missingno also serves as a nice item multiplication cheat; for whatever reason, when you catch Missingno it causes the item in your sixth bag slot to have 128 different ones (so if I had a Rare Candy in that sixth slot, Missingno would fly me up to 128). This creates a glitchy symbol in the bag where in displays how many of the item you have left, but when yon use 29 of the item if goes back to a normal 99 of course. This is nice if you want more valuable items such as Rare Candies and Master Balls.
You can find more info on Missingno on Bulbapedia here.
