The ISPICKVAL() function in Salesforce checks whether a picklist field has a specific value and returns TRUE or FALSE: ISPICKVAL(StageName, "Closed Won"). You need it because picklist fields can’t be compared with = in formulas and validation rules; Salesforce rejects Industry = "Banking" with a picklist error.
I tested the common patterns and mistakes in a Salesforce Developer Edition org with Check Syntax: comparing a picklist with =, checking for a blank picklist, and passing a field as the second argument. Below are the results, the syntax, and ready-to-use validation rules.