Impress is the slide presentation module of the OpenOffice suite. It is similar to Microsoft PowerPoint 2003. Users of MS PowerPoint would not have too much difficulty adapting to Impress's basic features and likewise, Impress users should be able to adjust to MS PowerPoint 2003 basic features with little difficulty.
All files end in a "file extension", usually a 3 or 4 letter extension. For example an Impress file ends in a 3 letter ".odp" file extension. Therefore every Impress document will end in this extension. (You won't see the file extension unless you turn on the Windows file extension view). MS PowerPoint 2003 ends in a ".ppt" extension. Even though both programs are slide presentation type programs, their different file extensions mean that they are different types of files. A common mistake is to assume that all slide presentation type programs can open each other's files. In order for one program to be able to read and open another program's file extension it needs to have a "filter" that does work of reading and opening that file. Without that filter programs can not read each other's files.
OpenOffice/LibreOffice uses a world standard Open Document Format (ODF), which means any program that supports the ODF format will be able to read and open OpenOffice/LibreOffice documents. However, Microsoft Office didn't support the ODF format until the MS Office 2010 version was released. What that means is that all MS Office programs below MS Office 2010 will not be able to open OpenOffice/LibreOffice documents.
Fortunately, OpenOffice/LibreOffice has built in filters that can read MS Office files. The filters work best on MS Office 2003 and below, but each new version of OpenOffice/LibreOffice adds new filters and purportedly are able to open the newer versions of MS Office. Impress can also save its files in the MS PowerPoint format with the .ppt extension. This is something to consider if you are emailing your Impress document to someone that only uses MS PowerPoint.
The Impress topics are shown on left under the heading "Impress Topics". Choose your topic, you will always know which topic page you are reading because the topic link will be white indicating you are on that topic.
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