Levels represent a user's contributions to the community. When a user creates a giveaway that's received by the winner, their level increases in value, allowing them access to additional giveaways as a reward. Levels range from 0 - 10, with level 0 being the default after registration, and level 10 being the highest level attainable. If you mouseover your level in the navigation bar, or the level on profiles, you'll see a tooltip with the level rounded to 2 decimal places, giving you a better idea of small increments in level, and when you're about to level up.
Levels increase almost exponentially, meaning each level requires double the contributions of the previous level. For example, level 4 would require double the contributions of level 3, and level 3 would require double the contributions of level 2.
How much a giveaway increases your level is based on the gifts price, so a $20 game would add twice as much value to your level as a $10 game. There are three other factors that may influence the value of a gift...
After you give away a game 5 times (whether in the same giveaway, or in multiple giveaways), it will begin decreasing in value. The value is decreased by 10% for each additional copy. This means if you decide to give away 8 copies of a $10 game, it would add $74.39 (10 + 10 + 10 + 10 + 10 + 9 + 8.10 + 7.29) in value towards your level. This has little effect on the majority of users, but prevents individuals from receiving a high level from gifting a large number of promotional gifts, games obtained through pricing errors, or keys from contacting game developers.
If a giveaway is invite only, or for a whitelist or Steam group, it will only add value towards your level if it receives 5 or more entries. This is in place to prevent small groups of friends from attempting to cheat the contributor system, and to encourage users to expand the reach of their giveaways. The whitelist feature offers an easy way to target a larger audience, and you can combine it with small Steam groups to receive a greater number of giveaway entries if needed.