Here is a light document, in the form of a presentation, to help IT architects or sales people to understand High Availability Clusters with Linux:

• How it works
• Components needed to build HA clusters
• Replication
• SCSI and Fiber Channel considerations
• Reference architectures
• Clustering of popular products
• Sizing guides
• etc

Start here and follow the links to browse it, download PDF or the original (ODF) file.

There are also excelent presentations and tutorials in the Linux-HA Project website.