Many BDSM relationships involve some sort of pain play. Typically one person enjoys giving pain and the other person enjoys receiving pain. Sometimes both people in the relationship enjoy being on both ends. These people are known as sadomasochist. Other times one person might enjoy giving or receiving pain, but the other person may have no interest in it, but still participates to make their partner happy. Often times sadism and masochism goes along with a D/s relationship, but the desire to cause pain or have pain inflicted on you is typically completely separate from any sort of power dynamic. What this means is that just because you enjoy being hurt does not mean you are submissive and likewise you can enjoy causing pain without being dominant. Whatever the case may be, these types of people are known as sadists and masochists. Let's break down each of these terms a little more.
Sadist: Merriam-Webster describes sadism as a sexual perversion in which gratification is obtained by the infliction of physical or mental pain on others. This means that a sadist is someone that enjoys causing pain, typically during or as a lead up to sex. This does not mean that there is something wrong with them. It's really no different than wanting to be kissed or touched a certain way. Most of the time sadists want a willing "victim." They don't want to cause pain to someone that doesn't enjoy it as much as they do. There are different levels of sadism. Some people just like to give a gentle spanking to their partners while others like to beat their partner until they bleed. Sadism often goes hand in hand with being a Dom, but not always. There are sadistic submissives or bottoms, but often these people consider themselves switches.
Masochist: Merriam-Webster describes masochism as a sexual perversion characterized by pleasure in being subjected to pain or humiliation. Masochists enjoy having pain inflicted on them. For some they can not enjoy sex without some sort of pain, that is how deep their desire goes. Just like sadists, their are different levels of masochists. Some can't handle a lot of pain and others have extremely high pain tolerances. Often times the longer you experiment with pain play the more pain you become able to take, but that is not always the case. Just because you may not be able to take as much pain as someone else does not mean there is anything wrong with you. Masochists are often the bottom in their relationship, but just like their are sadistic submissives, there are also masochistic Doms. Part of the reason for this is because just like I talked about in the first paragraph, while both sadism/masochism and dominance/submission may be included in a relationship, they are completely separate dynamics and not really connected in any way.
I hope after reading this article you feel more informed about the concept of sadism and masochism. Even if you decide it's not for you, I hope you learned something new.