What is the difference between cross-polarized yagi antennas and circularly polarized antennas?
Cross polarization usually refers to the opposite polarization. For example vertical and horizontal are opposites, as are right-hand and left-hand circular. Theoretically, a pair of cross-polarized antennas will have zero coupling, but in practice an extra 20 to 30dB of path loss is typical.
Usually this is an undesirable thing, but it can be exploited for diversity or multiplexing.
That said, I wouldn't be surprised to find lay usages of "cross polarization" meaning circular polarization. Many circular polarized antennas have elements arranged in a cross.