User variables find information from the current user's record in the People table and always begin with the
$global.my_ prefix. Saved searches and reports commonly use user variables for their data. For example, if a saved search reads "Company=$global.my_company" and the People table has a Company field, the field's value for the current user is inserted into the search condition when it's run. For a user who works at Acme, the resulting search condition would therefore be "Company=Acme."
When you are running a report, values from user variables are substituted at run time for each user so that a single report sent to multiple users contains the appropriate information for each user. For instance, Sonya can get a report of all the bugs that she submitted, while Jaime can get a similar report but with all the bugs that he submitted.
To see available user variables, go to the Global Variables tab of the Formula Help wizard.