Overview
Arc Grid coverage format is a proprietary format that is primarily used in ArcInfo. However GeoServer can serve it up. Wikipedia has a page that describes the Arc Grid format that will give you a good overview.
This page talks about the image format in detail that may put you to sleep.
Add an Arc Grid Coverage
Adding an Arc Grid Coverage to GeoServer is very easy.
1) Simply navigate to the Config->Data->CoverageStores->New menu:
2) Select the Arc Grid Coverage Format dataset option.
3) Enter the name you want to use for the dataset in the Coverage Dataset ID field. Then hit New.
4) You will be taken to the CoverageStore edit screen where you will have to enter the path to your Arc Grid file. You can enter a relative path to your data directory if that is where the file lives, or you can enter an absolute path to the location of the file.
For example: C:\my_data\rasters\canada.asc
The extension needs to be .asc
5) Hit the Submit button and save.
6) Next head over to the Config->Data->Coverages->New menu:
Select your coverage store and hit the New button.
7) You will now be in the Coverage Edit page. It will know your projection, so all you have to do it hit the Submit button at the bottom, then apply and save the changes.
You should now have your Arc Grid Coverage registered in GeoServer.
