Descriptive writing simply describes and explains. In contrast, critical/analytical writing:

• analyses how things work together, or how things are linked, or how things are similar and/or different
• evaluates strengths, weaknesses, relevance, value, effectiveness
• suggests new ways to explain things, new links between things, new theories, approaches or ideas

You can write descriptively and critically about pretty much anything, including events, people, procedures, decisions, research, ideas, theories, techniques, objects, and designs.