The Steem network has 4 accounts built into the network. Let's look at what they each are.
@miners
This account is the very first Steem account. This account can be accessed with the keys of any of the current active witnesses. So far, it has received transactions, but none of the active witnesses have ever made a transaction with this account.
@null
This account is an account that has no keys attached to it. It will never be able to spend funds. Additionally, anything sent to this account is destroyed. This account is used for promoting posts. Funds for promotion are sent to this account, so that they can't be used by anyone.
@temp
This account can be used by anyone, even people without a Steem account. Unfortunately, most Steem interfaces don't support using this account, as you need to pass no password to login, but most interfaces assume a password is needed. There are plans to make it so this account can be properly used to make accounts, after Hard Fork 20, as currently the way account recovery works with this account has some problems.
@initminer
This is the final special network account. The password for this account exists, but is only known by Steemit Inc. This account was used to make blocks, before enough witnesses joined the network.