.properties is a file extension for files mainly used in Java related technologies to store the configurable parameters of an application.

.properties files can use the number sign, '#', or the exclamation mark, '!', to denote that all text following it is a comment. An example of a properties file is provided below.

# You are reading the '.properties' entry in the English wikipedia.
 The exclamation mark can also mark text as comments.
website = http://en.wikipedia.org
language = English
 The slash below tells the application to continue reading
 the value onto the next line.
message = Welcome to \
    Wikipedia!

A .properties file maps a key with a value, it is essentially a hash table. In the example above, 'website' would be a key, and its corresponding value would be 'http://en.wikipedia.org'. Whilst the number sign and the exclamation mark marks text as comments, it has no effect when it is part of a property. Thus, the key 'message' has the value 'Welcome to Wikipedia!' and not 'Welcome to Wikipedia'. Note also that all of the whitespace in front of 'Wikipedia!' is excluded completely.