What Defines Role vs. Position.

For my first 30 levels, I've mostly spent my time trying a lot of different champs, bouncing between lanes and jungle to get a better understanding of the game. Having now hit 30 though, I'm looking to move into competitive, which means starting to solidify my role and champion pool. At first, I thought it'd be effective to simply scroll through the champion list and see what seemed interesting, but then I started getting confused about the designation of each champion. In other games, I tend to play a lot of tanks. In League however, what a tank is seems to be more abstract, since there are tanks in nearly every category. Similarly with supports, I'm not sure why champs such as Anivia are put in the "Support" role despite being played exclusively mid, or why Brand is a Mage, but is played as a support. Any clarification on how these identifiers work would be greatly appreciated. Thanks in advance.
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