they're basically tabular data where columns are homogenous in datatype, and where you can have different datatypes among the different columns. The rows are the data points or samples. They're very often thought of as lists of multidimensional data where the dimensions correspond to the columns, and the rows to the samples of the data.

This guy gives a good overview. https://github.com/mobileink/data.frame/wiki/What-is-a-Data-...