.CEL is the extension used by Affymetrix to name data files storing probe intensity measurements. Information about the array design characteristics such as location of probes, probe-sets and probe-pairs is stored by Affymetrix in CDF files.

In an experiment carried out using Affymetrix technology you will have a CEL file for each of the arrays you hybridized.

In Babelomics, CEL files are considered to be the raw data files of the Affymetrix platform.

Some CEL files can be in text format but most of them wil be in ASCII or binary format. Do not be too surprised if you cannot open them using a text editor or an spreadsheet.

Here you can find more information about Affymetrix CEL files format.

Information about some other Affymetrix data files can be found here.