Here are few good metalink notes to understand Oracle Apps Patching (adpatch), You need Metalink Account to access these notes. 

181665.1 – Release 11i Adpatch Basics

457566.1 – Oracle Applications Patching FAQ for Release 11i

459156.1 – Oracle Applications Patching FAQ for Release 12

60766.1 – 11.0.x : Patch Installation Frequently Asked Questions

225165.1 – Patching Best Practices and Reducing Downtime

314540.1 – What Log Files are Generated with Adpatch, and Where are They Located?

175485.1 – How to Apply an 11i Patch When adpatch is Already Running

136342.1 – How To Apply a Patch in a Multi-Server Environment


Here are few more from our Author – Subba Rao

342035.1 – How to Show the Patch History for Merged Patches in OAM

228779.1 – How to Merge Patches Using admrgpch

432597.1 – Reducing Down Time When Applying Patch for Oracle E-Business Applications 11i

231701.1 – How to Find Patching History (10.7, 11.0, 11i)

235447.1 – Application Install AutoPatch Setup Test