Amazon uses zones that look like this: us-east-1d. What do the letters of a,b,c,d,e represent?
The letters actually show the Availability Zone: in us-east-1d notation, us-east-1 is the Region's ID, and d is AZ's identifier.
For example, US West has (at this very moment) two Regions assigned: us-west-1 (Northern California) and us-west-2 (Oregon). I'd suggest checking Regions and Availability Zones docpage for further details.
Note that availability zones are assigned per account:
To ensure that resources are distributed across the Availability Zones for a region, we independently map Availability Zones to identifiers for each account. For example, your Availability Zone us-east-1a might not be the same location as us-east-1a for another account.
... so 'a', 'b', etc. are logical, not physical identifiers of Availability Zones.