It isn’t uncommon to see hackers breaking into a database and then they post all the stolen credentials (usernames and passwords) on public websites making them accessible to anyone and everyone.
PwnedList is an online tool that monitors such leaked lists of compromised accounts and will alert you as soon as it finds your online account – the email address associated with your login – in any of these databases.
The service mostly sources data from from account dumps made by various hacker groups though they also spider underground websites to identify potential account dumps and imports them without human intervention.
One more thing. If you discover that your email address is part of any of these account dumps, it may not necessarily mean that your account has been compromised. That’s because the hacker groups would still need to decrypt your passwords before gaining access to your account.