A trundle bed is a bed that slides under another bed. It is pulled out as needed to additional sleeping space, say, for an overnight guest. A daybed is a bed that is not designed as a traditional bed. It often looks like a large sofa, with a back and arms, as opposed to a headboard/footboard configuration. A daybed can be used in an office or den or sun porch for extra seating during the day. Any bed can have a trundle bed. A daybed can have a trundle bed, which is often hidden by a long dust ruffled. A daybed is one thing; a trundle bed is another. A daybed may or may not have a trundle; it is in any case a daybed all the same. A day be is a twin sized bed that is made to look like a couch. A trundle is often a option for a day bed and in a mattress that slides under the main bed when not in use.