Orca is a tool to edit entries of MSI files.
It's very useful to inspect msi files and to make some small changes in msi files (modify or add table rows). The simplest case is changing a property that affects the behaviour of the installer or of the installed product, for example to disable the included automatic updater.
Orca allows to either edit a MSI file directly, or to store modifications as MST files. To do this, select "New Transform" from the Transform-Menu before doing any changes. After applying all changes, select "Generate Transform". Orca will then ask for the name of the new transformation file.
Orca is available for free from Microsoft, but they don't offer to download just this tool separately. They only include it in other downloads:
By installing one of those packages, Orca is not automatically installed. Instead after the installation search for a file named orca.msi. Install that to get Orca. You might also be able to extract orca.msi from the downloaded file with a tool like 7zip, thus get it without having to install the full package. With the Installer package first extract the CAB, then open that (e.g. also with 7zip) to find orca.msi.
To use orca, simply drag the msi file onto the program. Please note that this will always at least change the date of that file. You should usually work with copies of the MSI files.
Microsoft offers some help for using Orca on this page:
MSI files can also be modified with vbs-scripts, but for that you must already know precisely the change that you want to apply, because this does not allow to inspect the file. You could use Orca to find and test a modification to a specific software product, and then write a vbs script to automate that modification for future versions of the same product.