.box - Used for mail routing. Mail is stored either in the client's or server's mail.box before being routed to the recipient.
.dic - User dictionary file (obsolete as of 8.5.x Standard if using the new spell check engine)
.dll - Dynamic-link Library
.dmp - Dump file, usually created during a crash.
.exe - Executable file.
.ft - Full Text Index
.gif - Type of an image
.hst - History file, Example fault_recovery.hst
.id - A Notes/Domino ID file, belonging to a user, server, or a certifier ID
.ini - Configuration settings file (INI = Initialize). These files are most commonly read at launch time.
.jar - Java class package file
.jpg - JPEG image
.lck - A file which locks another. Usually you will see a file with its normal extension and another with .lck if the file is in use or locked for other reasons
.lock - Same as .lck
.log - Non-Eclipse log file. Usually used to troubleshoot core client issues (ones that would be seen in the Basic client, like console logs and NSD files). Eclipse logs are in .XML format.
.nbf - Process table file
.ncf - Files similar to databases but are not read by humans. Examples include ~editclp.ncf (Notes clip board file) and cluster.ncf (File which contains cluster information)
.ndk - Another file similar to .ncf. Examples include Cache.NDK (Client cache) and Desktop8.ndk (Where workspace icons, some cached design elements among other things are stored)
.nsf - Notes System File, a Notes/Domino database.
.ntf - Notes Template file, a Notes/Domino template.
.prefs - A preference file, commonly found in
.properties - Similar to ini files but in the Eclipse side. Example, rcplauncher.properties, a file read by rcplauncher.exe when Notes2.exe is run.
.txt - A text file
.xml - An XML file. Most commonly used for Eclipse logs (Found in Data/Workspace/Logs)
I got this information from the TN1675947