Microsoft PowerPoint uses a dictionary of common words to check the spelling in a presentation. This dictionary is shared with products such as Microsoft Word and Microsoft Excel. The spelling check will stop on words that are misspelled or aren't in this dictionary, and you have the option to add the words to a custom dictionary at that point. The custom dictionary is handy for specialty terms, such as medical terms or words unique to your work environment.
To broaden the type of text you can check for spelling, PowerPoint uses dictionaries of other languages as well as supplemental dictionaries when installed by other programs in Microsoft Office.