The most important signatures and parameters of stellar magnetic activity and their relation with global stellar parameters are briefly reviewed with the aim of indicating which are the most significant data that are required to constrain possible models of stellar activity. The sporadic and cyclic variability aspect is, particularly stressed, firstly, because it is a crucial activity parameter and, secondly, because of its possible effects. on the derived general correlations between activity and stellar parameters. In particular, the rotation rate, although it is recognized to play a relevant role, still its correlation with various activity indicators has an essentially qualitative character, because other global stellar parameters undergo concurrent changes, making it difficult to isolate the pure effect of stellar rotation from observations.