This information can be found on Apple's site at:
10.1.5 Features/Enhancements --
Enhancements Delivered with this update
In addition to enhancements previously delivered with Mac OS X 10.1.1, 10.1.2, 10.1.3, and 10.1.4, the Mac OS X 10.1.5 update includes the following:
Networking, Mail, and Internet Connectivity Enhancements
*\tA mail account will go offline if a secure connection to a remote SSL mail server cannot be made (when the SSL encryption option is enabled).
*\tImproves robustness when accessing files and directories on Microsoft Windows-based servers.
*\tResolves a potential Mail issue in which mailbox draft messages (besides the first) could be inadvertently deleted when using Rebuild Mailbox.
*\tEnhances the security of sendmail (a mail function in the Darwin subsystem unrelated to Mail.app).
*\tImproves compatibility when accessing a WebDAV volume via a non-standard http port.
*\tImproves function of certain kinds of WebDAV DNS lookups via an AirPort Base Station (Graphite).
*\tPermits communications with WebDAV servers that use long http header lines.
*\tReduces time required to list iDisk and WebDAV server directories.
*\tImproves buffer efficiency during WebDAV downloads.
*\tImproves WebDAV compatibility with servers that use chunked transfer-coding.
*\tAddresses a potential kernel panic situation that could occur when saving a file stored on an iDisk or WebDAV volume.
*\tTraditional Chinese text now appears when expected within Internet Connect (AirPort configuration).
*\tImproves general reliability of Mail.
*\tAdds a man page for mount_webdav (man pages are accessible when using the Terminal man command).
Device Compatibility Enhancements
*\tIncludes newer Adaptec SCSI card drivers (version 1.1.0) for Apple Ultra SCSI PCI cards and Apple 2906 SCSI PCI cards.
*\tAddresses an issue in which a SCSI-based magnetic-optical drive may inadvertently report media as write protected when it is not.
*\tImproves compatibility with certain Canon digital cameras.
*\tImproves reliability when deleting multiple images from certain Nikon digital cameras.
*\tImproves reliability of audio applications when a USB MIDI device is disconnected and reconnected.
*\tImproves PC card (generic serial communications) support, enhancing potential compatibility with third-party PC slot communications cards.
*\tReduces likelihood that a non-recommended display resolution and refresh rate could be chosen (resulting in a persistent black screen), when using certain Power Macintosh G4 (QuickSilver) computers with an AppleVision 750 or AppleVision 1710 display.
*\tImproves compatibility of Power Macintosh G3 (Desktop and Mini Tower) computers with ATI Radeon 7000 video cards.
*\tAdds support for LaCie Pocket CD-RW devices 16-10-40 U&I, and 8824 U&I
*\tImproves iSub frequency cutoff range.
*\tResolves a potential solid screen color issue, when using an nVidia GeForce 3 video card in a computer with 1 GB of RAM installed.
*\tAddresses various other potential nVidia video card issues.
*\tWhen using certain USB speakers, a mute setting is preserved more reliably after waking from sleep.
*\tReduces the time required to open optimized applications that use embedded frameworks.
*\tAddresses a potential stall issue that could occur while iTunes was finishing a CD burn.
*\tEnhances stability of Sherlock.
*\tAdds Image Capture support for certain recently-introduced Canon USB cameras.
*\tAddresses an isolated issue in which a Hanja text conversion could fail when using Korean Input Method.
*\tImproves Korean Input Method (Hanja) compatibility with AppleWorks 6.2.
*\tImproves Korean Input Method (Hanja) compatibility with Adobe Photoshop 7.0
*\tfsck_hfs (file system check) now works with Mac OS Extended format volumes larger than 1 TB.
*\tAddresses various potential Apple DVD Player issues.
Finder, Sleep, and System Settings Enhancements
*\tEnergy Saver now offers an advanced option, on compatible portable computers, to increase battery life by reducing processor performance while the computer is not used.
*\tResolves a potential issue in which audio may have become muted after waking from sleep.
*\tThe Sound preference pane may now be opened by pressing a Option-Volume key combination on an Apple Pro USB Keyboard.
*\tAddresses a potential situation in which the Finder may inaccurately report less available space on a blank disc, after ejecting a disc instead of burning new data to it.
*\tImproves performance of QuickTime movies played on a computer that uses an ATI Rage Pro graphics acceleration chip.
*\tAddresses various potential kernel panic situations.
*\tAddresses various potential video mirroring issues.
I thought this list did a good job of summing it all up in plain English and answered many users questions posted in this area.