When an admin posts on a user's wall telling a user to correct something they did or to stop doing something, it is a notice. Notices are the least severe type of penalty and usually will have little impact on a user.
Warnings are a more severe type of penalty, meaning that a user has broken multiple policies or has continued to break the rules. Not following up to a warning or continuing to break the rules will result in a block.
A block is when a user has failed to follow up to a warning or are preforming multiple severe violations of policies in a short amount of time. A user might be blocked instantly for vandalizing the wiki. Blocks can be temporary or permanent depending on how severe the issue is.