pKa is an acid dissociation constant used to describe the acidity of a particular molecule. Its value is directly related to the structure of the given compound. The constant changes depending on the solvent the compound is used in. Typically, organic chemists compare the various values from their determination in water, DMSO and the gas phase and use these to predict a compounds reactivity, solubility, and other physical characteristics. More information on pKa's can be obtained elsewhere (e.g Wikipedia pKa). In short, the stronger the acid, the smaller the pKa value and strong acids have weak conjugate bases. pKa values describe the point where the acid is 50% dissociated (i.e. deprotonated). Below are tables that include determined pKa values for various acids as determined in water, DMSO and in the gas Phase. These tables are compiled in PDF files below.

For a more comprehensive discussion on this topic, please see Acidity and Basicity by professor William Reusch, Michigan State University.