Cell biology (aka cytology) or cellular biology is mostly about the biology of cells in a general sense. It is about investigating how the various processes and cell structures within a cell are interlinked. It often uses the techniques of biochemistry and molecular biology but on a more cell wide scale.
A typical eukaryotic animal cell. image: wikipedia
A typical area of study would be investigating how a cell divides, and considers all the various structures (like chromosomes) and steps for a cell to divide. It considers the differences and similarities between different types of cells.
Traditionally cell biology relies on the use of the microscope (from , light to electron microscopes) to investigate various structures of different types of cells.
It is about asking questions such as: What are the similarities/differences between a bacteria dividing and animal cell dividing? Then it would look at the molecular biological differences.
Further reading: