A blockchain address, including those for bitcoin, is usually a placeholder to accept bitcoin transactions on a blockchain. It is simply a number to send transactions to. Along with a private key, an address is a secure identifier. Bitcoin and most other cryptocurrency addresses are different from other internet addresses in that they are only used once. A bitcoin transactor's wallet automatically generates a new address for each transaction on the blockchain.
Bitcoin addresses are usually 34 bits long and include digits 1 to 9 and both uppercase and lower letters, except uppercase letters "O" and "I" and lowercase "l". Addressing is case sensitive. When shorter addresses are observed, it is usually because one or more zeroes have been admitted from the beginning of the address. 
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- Address. bitcoinwiki.