They are similar. Data frames are lists of vectors of equal length while data tables (data.table) is an inheritance of data frames. Therefore data tables are data frames but data frames are not necessarily data tables. The data tables package and function were written to enhance the speed of indexing, ordered joins, assignment, grouping and listing columns (etc.).
See http://datatable.r-forge.r-project.org/datatable-intro.pdf for more information.
