Bitcoin addresses beginning with 3 are a multisignature address. This type of addresses is used by the service in a 2-of-3 configuration. It takes two signatures to transfer bitcoins.
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I realize that my answer is probably more technical than the OP is ready to understand, but since this is the "
Technical Support" board, I'd just like to mention that bitcoin addresses that start with a 3 are "Pay-to-script-hash" (P2SH). One of the most common uses for a P2SH transaction is to implement an M-of-N multisig requirement (such as 2-of-3 or 3-of-5, etc). However, it is possible to have an address that starts with a 3 that is not a 2-of-3 multisig (or M-of-N multisig).
Additionally, addresses that start with a 1 are "Pay-to-public-key-hash" (P2PKH).
Note that at the protocol level there are no "addresses" at all. There are simply requirements that an output is encumbered with by way of a script that must be satisfied in order to use that output in the funding of a transaction. "Addresses" are just a shorthand (and user friendly) way to indicate to the wallet what type of script to use (along with a checksum to allow the wallet to double-check your typing for mistakes). The first 8 bits (first byte) of every address is a "version" number that tells the wallet software which type of script to use to build the transaction.
A byte of 0x00 indicates that the wallet should build a P2PKH transaction. When converted to Base58Check notation, any leading 0x00 is converted to a 1.
A byte of 0x05 indicates that the wallet should build a P2SH transaction. When converted to Base58Check notation, a leading 0x05 byte will result in an address that starts somewhere between '51' (for a script hash of 0x0000000000000000000000000000000000000000) and '5R' (for a script hash of 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF).