A spammer is a person or group that sends you email you don't want and didn't sign up for. The email a spammer sends is called spam.
You probably delete everything spammers send you without even opening the email. Spam is usually some kind of advertisement, and even though you may not be tempted to click, it's a cheap way to advertise, and it remains difficult to track down spammers and hold them accountable. The words spammer and spam come from the canned meat called Spam and the Monty Python comedy sketch that was inspired by it.