All accounts owned by an individual may be restricted for any violation of the Steam Subscriber Agreement or Steam Community Guidelines including but not limited to:


Piracy or Hacking;
Payment Fraud;
Redeeming Fraudulent Gifts;
Hijacking or Sharing Accounts;
Account Theft and other Deliberately Deceptive Activity;
Buying, Selling, or Trading Accounts;
Scamming, Impersonation, or Social Engineering;
Abuse and Harassment;

Account Locks are issued for various reasons related to violations of the Steam Subscriber Agreement. Some locks are intended to freeze an account in its current state until the account's owner contacts Steam Support to resolve the issue, and others are intended as temporary or permanent restrictions applied in response to a violation of Steam's rules. Any account lock's notification message will tell you what's restricted and why.

Community Bans are issued to an account when the account is used in ways that violate Steam's Community Guidelines excessively. Most Community Bans will expire, but extended or even permanent bans may be issued if the prohibited activity continues or if the moderation team finds the activity egregious.