A circuit is a handful of daily puzzles played as one sitting and ranked on your combined placement across all of them, shortest game first. The Daily Five changes every day. The rest are fixed, so you can pick the skill you want to work and come back to it tomorrow.
Every game is free and needs no account. A game played on its own still counts toward its circuit, but all of them have to land on the same day to take a rank on that circuit’s board.