In a BDSM relationship the partner who has the active (i.e. controlling) role in a session or in the entire relationship is described as the “top”. The partner referred to as the “bottom” submits voluntarily to the actions of the top. Often the terms “dominant” and “submissive” are used interchangeable with “top” and “bottom”.
A switch is someone who participates in BDSM activities sometimes as a top and other times as a bottom or (in the case of dominance and submission) sometimes as a dominant and other times as a submissive. Partners may switch roles based on mood, desire, or to allow each partner to experience their preferred activity. For example, a switch may be in a relationship with someone of the same primary BDSM orientation (e.g., two dominants), so switching provides each partner with an opportunity to realize his or her BDSM needs. It is also common for people to switch with different partners, such as when a person acts exclusively as a top with one partner and exclusively as a bottom with another.
In all cases, the common theme is one of consent. Regardless of which role you take on, getting ongoing and enthusiastic consent is paramount.