There is first and foremost the Kt/V equation, the currently accepted standard for determining dialysis adequacy. K here is the dialyzer blood water urea clearance, and in practice one calculates the Kt/V for dialysis patients by measuring their blood urea concentration both before and after dialysis.
Next we have the K(oa), the dialysis mass transfer area coefficient, which describes the efficiency of a given dialyzer in terms of its ability to remove solute. The K(oa) may be thought of as the maximum possible clearance by a dialyzer at infinitely large blood and dialysate flow rates. K(oa) values are traditionally reported in cc/min; standard dialyzer K(oa) values are typically 500-700 cc/min with "high efficiency" dialyzers (not to be confused with "high flux" dialyzers--see below) having K(oa)'s > 700 cc/min.
The K(uf) is the ultrafiltration coefficient, and it describes the ability of a given dialyzer to remove fluid at a given transmembrane pressure gradient per unit time. Dialyzers with a high K(uf) (e.g., >20 cc/hr/mmHg) are considered "high flux" dialyzers, and these may be especially helpful in patients with large intradialytic weight gains.