This Sigil of Baphomet is often used as a symbol for Satanism

Satanism is a word that has several different meanings. It can describe different religions and philosophies.

The Cross of Saint Peter, an upside down Latin cross, is also used as a symbol for Satanism

Sometimes the word Satanism is just used to say that the person using it does not like a group, but the group themselves do not think of themselves as Satanists. Used like this, it is just used to say the speaker disapproves of a group. A lot of groups, like Pagans, or even Protestants have in the past been called Satanists by some people.