Most of the time you will receive emails that you have signed up for in the past. You may not want them anymore but they are not spam as you originally asked for them. To stop receiving these, click on the unsubscribe link which is usually at the bottom or top of emails that have them.
Occasionally you will receive offers, promotions and other emails where you do not remember signing up. If this happens to you, do not use the unsubscribe link. This is because these are spam messages used to try and get your details. Sometimes it is just to see if your email address is genuine. A good give-away for this is if your name is in the CC of the email (for example John Smith) but you can also see John A Smith, John B Smith, Jonnie smith etc. in there. They are trying every combination they can think of to get a hit. Other times it is to get other information such as a username or password. If you have different passwords for every website this is not a problem but a lot of people keep the same password for many sites. Once they get hold of that password, they have access to all the websites which use the password and this could be banks or stores.