A cell is simply a list of identifiers (such as customer or prospect IDs) from your database. In Campaign, you create cells by configuring and running data manipulation processes in flowcharts. These output cells can also be used as input for other processes in the same flowchart (downstream from the process which created them). There is no limit to the number of cells you can create.
Cells to which you assign one or more offers in Campaign are called target cells. A target cell is a distinct group of homogeneous individuals (or whatever the entity is for which the audience level is defined, such as individual customers, or household accounts). For example, cells can be created for high-value customers, customers who prefer to shop on the web, accounts with on-time payments, customers who have opted to receive email communications, or loyal repeat buyers. Each cell or segment you create can be treated differently, receiving different offers or communications via different channels.