Perhaps the most important basic distinction in evaluation types is that between formative and summative evaluations.
Formative evaluations seek to examine how and why your programs and services work, for whom they work, and in what circumstances. The formative evaluation informs on program progress to date, as well as to identify areas for potential improvement and additional focus. It helps you with the planning, setting up, and carrying out of a project, as well as the documentation of the evolution of a project.
Summative evaluations, in contrast, assess the short- and long-term results of your project by describing what happens after delivery and assessing causation between the program and observed outcomes.