Custom Label: 

These are custom text value that can be accessed from Apex classes or Visual force pages. These values can be translated into any language Salesforce supports. 

Custom labels enable developers to create multilingual applications by automatically presenting information (for example, help text or error messages) in a user’s native language. 

Limitation

You can create up to 5,000 custom labels for your organization, and they can be up to 1,000 characters in length.

How to access custom label in visualforce page:

{!$Label.demolabel}

How to access custom label in Apex classes:

System.Label.Label__Name;

For In my example:

System.Label. demolabel