The ability to place a card (or group of cards) on another card. In regards to rank, you can BUILD UP, BUILD DOWN, or BUILD UP/DOWN BY *. In regards to suit/color, you can BUILD BY SUIT, BUILD BY COLOR, BUILD BY ALTERNATE COLOR, BUILD BY ANY SUIT BUT OWN, or BUILD REGARDLESS OF SUIT. Note that all games that build will follow two of these rules, one from each list.
Adding a card to a pile. Building up means playing the cards in ascending order, building down means playing the cards in descending order. Building can follow suit, color or alternate colors, but there are games that build up and down regardless of suit or color.