A merchant account is a type of bank account opened by an acquiring bank (also called acquirer). It allows businesses to accept card payments and perform other card related operations (such as authorizations, refunds, resales, etc.).
Access to a merchant account is usually provided by a separate institution (payment gateway/provider). It can be considered "an interface" to a merchant account and usually provides a number of other payment services. Please note that merchants do not have direct access to merchant accounts - these are just meant to be used with card operations. Funds stored on a merchant account are paid out within specific time periods to a regular company account (as specified in an agreement with the merchant).