Regardless, write your abstract using concise, but complete, sentences.
The researcher evaluates the paper and often compares it with other works on the same subject.
Critical abstracts are generally 400-500 words in length due to the additional interpretive commentary. A descriptive abstract indicates the type of information found in the work.
Descriptive abstracts are usually very short, 100 words or less. While they still do not critique or evaluate a work, they do more than describe it.
A good informative abstract acts as a surrogate for the work itself.