A Grading Service, sometimes called a third-party grading service or TPGS, is an independent company that will grade and authenticate coins for a fee.
Fees are often tiered so that it costs more to grade rare or valuable coins than more widely available coins.
Many graders offer additional services like attribution of die varieties or high-resolution photography for an extra charge.
Once a coin is authenticated it is assigned a grade and encapsulated in a tamper-proof plastic casing.
Because anyone can advertise themselves as an evaluator, it's important to use a trusted professional grading service.