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"On an iPad that supports it, you can dictate text instead of typing. To use dictation, Siri must be turned on and iPad must be connected to the Internet. You can include punctuation and give commands to format your text." — iPad User Guide, p.26
Note that Dictation may not be available in all languages or in all areas:
And to see which iPad models support Siri:
There are also apps available such as Dragon Dictation.