Cell characterization is a process of analyzing cell design, normally extracted as spice circuit, to generate cell models which captures cell timing, power, etc. information and can be used in backend process such as synthesis, timing closure, layout, power analysis, SI analysis, etc.
Two good topics about this topic.
Page 1 is a Master thesis written by Jos Budi Sulistyo of VT at 2000. Concept still applies to today’s design.
Page 2 is slides from Silvaco.