A Mii is a digital character or 'avatar' on your Nintendo Wii or Nintendo 3DS systems. You can change the features of your Mii as you see fit, most people create Mii versions of themselves. The Mii is then capable of being used as a playable character in several games on both consuls, such as Wii Sports, Wii Fit and Mario Kart. Mii's are transferable from console to console through storage on the Wii mote but are only editable on their original console. Mii's are also capable of mingling with other Mii's via the WiiConnect service.
Creating a Mii is simple, it can be done through the Mii channel on the Wii. You will first have to specify the gender of the Mii then the user will get to adjust the facial features of the Mii and determine a body shape for your Mii. You can then adjust the "birth" date of your Mii, it's favorite color and give it a name. The limitations of the graphics cause the Mii's to be more like caricature than actual self portraits.
A Mii takes having an avatar to the next level by allowing you to actually use your Mii in several different games. One of the original titles that used the Mii was the original Wii game Wii sports. You could use your Mii to play baseball, play tennis, bowl, play golf and to box. By using the Wii mote and your Mii you were able to build up skill and earn your Mii the title of Pro in each of these sports individually. This was done to help showcase the initial potential of the Mii on Nintendo's Wii system. The Wii was initially marketed to be a gaming system for everyone rather than just those in society known as gamers. The Mii was part of this concept allowing a higher level of absorbtion in the gaming process for those that have never done it before. The use of the Mii plaza and other such networking features allowed users to get more out of their systems than just games and thus opened the Nintendo up to several new markets.
Having a Mii is great, it allows you to play your self in several Wii and Nintendo DS titles and socialize with other users. They can represent you or someone else and can even have their own identity. You can take your Mii with you between several Nintendo devices, meaning your Mii will probably be with you as long as you are using a Nintendo system. The Mii is the ultimate avatar in the world of Nintendo, and can provide you with the ultimate gaming experience on Nintendo's Wii system.