Parasitsm - a relationship in which one organism is harmed and the other benefits.
Parasitic friendships are just that. When your friend benefits, but when you are harmed. Or it could be the other way around: You could benefit and your friend could be harmed. Either way, people are getting harmed. It's okay to be friends with people who's ethics and religion may be somewhat different from yours, but if they start changing you for the worst, it's probably not a healthy relationship for you to be in. Likewise, if you realize that you are badly influencing a friend, you probably need to back off.
Commensalism - a relationship in which one organism benefits and the other neither benefits nor is harmed
Commensalistic friendships are A-okay to be in. They're not, persay, the friendships that everybody dreams of in which each of you would give your life for the other, but they're perfectly fine. As long as the relationship is not having a negative effect on you or your friend, than it is alright. But remember, you want to benefit from a relationship. If you're not being benefited, why stick around? I know that sounds stuck up, but you really don't want that person's neutral effect to change to a negative one.
Mutualism - a relationship in which both organisms benefit
Mutualistic friendships are the best kind of friendships. Each of you is shaping the other into a better person. You are honest with each other and hold each other accountable for your actions. Mutualistic friendships are the ideal friendship that everyone should strive for.
Think about it. Are you benefitting or harming your friends. Are your friends harming or benefitting you? These are all important factors in choosing your friends and deciding what kind of friend you should be.