What is Club Nintendo?
Club Nintendo is a free service provided by Nintendo allowing you to earn rewards for buying games on Nintendo platforms. There are several versions of this service in different regions (Japan, US, Europe and Australia being the main ones). Each region only accepts games purchased within the region, so codes are generally not exchangeable (although some selected European codes will work for the Australian service and vice versa). Both retail games as well as games available on the various console-based stores on Nintendo consoles qualify for rewards.
The benefits you receive from Club Nintendo come in several forms. The currency of Club Nintendo is coins. You receive coins for filling in the surveys you are given when registering the purchase of eligible games (see Earning Coins). These coins can be used to buy Nintendo-themed prizes (see Rewards). If you earn a certain quota of coins in any Club Nintendo year (which runs from July 1-June 30), you will also receive a special free bonus prize (see Elite Status). Being a member also qualifies you for certain benefits from time to time, such as discounts on Nintendo concert tickets and warranty extensions on registered hardware.
I'm joining for the first time and I have a lot of past codes to enter. What's the best approach?
Read this post first and learn all the mechanics. You may want to stop registering games after you have earned enough coins to reach Platinum Elite Status for the year (keep post play survey values in mind if doing this). You can leave the extra codes to make it easier to reach Platinum in future years too.