How to undertand the STA.RTP file

Hi, Can any one tell me how to comprehend the STA.RPT file. Specifically I do not understand the Hold times and setup times listed. For setup times rise and fall times are showing up as negative values.

; Hold Times
; Data Port ; Clock Port ; Rise ; Fall ; Clock Edge ; Clock Reference

; Setup Times
; Data Port ; Clock Port ; Rise ; Fall ; Clock Edge ; Clock Reference

Thanks in advance.
AlteraUser99