data, datum

Use data as both a singular and plural noun. Don't use datum.

Always use data with a singular verb, even if you're talking about more than one data set. Don't use the data are.

Lowercase data unless it's at the beginning of a sentence or phrase.

Examples
The data shows that 95 percent of users prefer a graphical interface.
The data gathered so far is incomplete.
These facts contradict earlier data.

See also big data