Steam is the primary video game distribution platform in the PC market. They sell thousands of games and offer online connectivity with your friends, community message boards, mod support for many games, a community market place, and the Steam Wallet. The steam wallet is essentially what it sounds like. It's a digital wallet you can deposit money into through gift cards, money transfers, or selling game items on the community market place. The money in your Steam Wallet can be spent on anything on Steam including games, software, movies, dlc, and in-game items. For this reason, the redeemable codes found on the back of the Steam gift cards are a hot trading item, both among the Steam community and other internet hubs. Many people trade the Steam Wallet codes for things like in-game items unavailable on the marketplace or player services for multiplayer games.
The demand for Steam Wallet codes has also sparked many different websites and communities offering the codes for free. Some legitimately, and some not. One example of a commonly used scam is know a Steam Code Generator. These are downloadable programs claiming to be able to generator working Steam Wallet Codes. Often times, these programs are actually just viruses or malware. Another example are "hack sites" which are websites promising to send you working, "hacked", Steam codes in exchange for money. These are usually scams with the intent to just take your money and send you a fake code. Any code that is real would likely be stole and could put your Steam account at risk.
While many scammers have popped up, there have also be many legitimate websites offering free Steam Wallet codes in exchange for simple tasks. A popular form of website, known as "Survey Sites", are among the most common websites known to do this. Survey sites typically offer rewards or money in exchange for completing a few online surveys. Companies are constantly looking for new feedback for their latest products. Many of them will offer you something for your time and opinions. While not all of these websites offer Steam Wallet codes as a reward, many have been known to. Other sites, known as "Ad Sites", offer a similar deal. Instead of giving their opinion, the user is asked to watch commercials and advertisements instead. Occasionally, they'll ask for a sentence or two of feedback but it's mostly just watching.
Being able to tell the difference between a scam and a deal is crucial to staying safe on the internet. Stay away from anyone offering something for nothing.