Abstracts are the purely descriptive summaries often found at the beginning of scholarly journal articles.
Annotations are descriptive and critical, describing the author's authority, point of view, as well as the scope and purpose of the source.
The University of Toronto offers an example that illustrates how to provide a summary of a study's research methods and argument.
The Memorial University of Newfoundland presents these examples of both descriptive and critical annotations.
Cornell University Library offers these examples of APA formats for descriptive bibliographies.