If looking at a graph of how many times the verb coming after these two words is confused, the numbers would be off the chart.
Datum means a lone, singular bit of measured information. Data is the plural of datum, as in Data about parking violations in New York City is available online.
Because of this, data technically requires a plural verb, as in The data are irrefutable. Of course, this sounds odd. Data seems to be a collection that is considered a single, unified object, in the same way that the United States is singular rather than plural.
My advice: Know your audience. If writing for a formal, academic purpose, show off your smarts by going with a plural verb. In all other instances, be a normal human being and use a singular verb.
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