In the context of the exam questions, this means that: you are required to describe the similarities and/or differences between things, not just write about one.
It is a very common mistake in exams to consider only the characteristics of one thing when this command word is used in the question. For a valid comparison you need to show you are aware of the features of BOTH things being compared. Make sure to mention each, outlining what they have in common AND how they are different.
A good technique would be to consider each similarity or difference in turn. Take care to mention both things being compared and use comparative terms such biggER / smallER / MORE / LESS or to say that one has something whilst the other does not.
For example, if the question is to compare meiosis and mitosis, you could write the following:
Both meiosis and mitosis are forms of cell division. Meiosis involves 2 divisions and produces 4 daughter cells. Mitosis involves only one division and produces 2 daughter cells. The daughter cells in meiosis are all different but the daughter cells from mitosis are all identical. Meiosis only occurs in the gonads and is only used to produce sex cells (gametes). Mitosis occurs throughout the body and is used to produce new cells for growth and repair. The daughter cells from meiosis are all haploid whereas the daughter cells from mitosis are diploid.
Lots different structures and processes can be used in this type of question. Typical examples worth preparing in advance are:
- Communicable and non communicable diseases
- Arteries, veins and capillaries
- Prokaryotic and eukaryotic cells
- Plant and animal cells
- Transpiration and translocation
- Light and electron microscopes
- Xylem and phloem
- Sexual and asexual reproduction
- Biotic and abiotic factors