A keypunch is a device to punch the holes in the card, not the card itself. Punched cards were an early method of storing and inputting data into a computer. A "deck" of punched cards would be placed in a hopper which would then proceed to read the data from each card and is pretty much synonymous with reading a file from your hard drive today.
A mainframe is a central computer where all the processing and storage is done centrally and accessed via a terminal. A terminal is a dumb device which generally has no local processing or storage capability and consists pretty much of a keyboard for input and a monitor to display output.