What is the output type of the 74AHC594?
It is a CMOS output. The specifications are in table 6 in the datasheet:




  There is stated in some datasheets for the 74594 IC from other manufacturers that the output type is buffered. What does that mean?


The specifications of 74594 aren't relevant. Those are for the original 74xx logic family, which was a TTL family. Your part is in the AHC family, which is CMOS. 

If the part had buffered outputs, that meant it was able to drive or sink more output current than un-buffered parts in the same family.

Note: Your datasheet also covers 74AHCT594, which is a CMOS part with TTL-compatible inputs. If you don't want TTL compatible inputs, don't use this part and don't worry about its specs.