Data archives may provide guidelines on how to cite the data, e.g.,:
Bachman, Jerald G., Lloyd D. Johnston, and Patrick M. O'Malley. Monitoring the Future: A Continuing Study of American Youth (12th-Grade Survey), 1998 [Computer file]. Conducted by University of Michigan, Survey Research Center. ICPSR02751-v1. Ann Arbor, MI: Inter-university Consortium for Political and Social Research [producer and distributor], 2006-05-15. http://dx.doi.org/10.3886/ICPSR02751.
ASTER Global Digital Elevation Model, version 1, ASTGTM_N11E122_num.tif, ASTGTM_N11E123_num.tif, Ministry of Economy, Trade, and Industry (METI) of Japan and NASA, downloaded from https://wist.echo.nasa.gov/api/, October 27, 2009
Sidlauskas B (2007) Data from: Testing for unequal rates of morphological diversification in the absence of a detailed phylogeny: a case study From characiform fishes. Dryad Digital Repository. doi:10.5061/dryad.20. Accessed August 15, 2011.
Tables, charts, graphs, maps or figures appearing in a publication
United States. Bureau of the Census. "Table 6. People with Income below Specified Ratios of their Poverty Thresholds by Selected Characteristics: 2009." Income, Poverty, and Health Insurance Coverage in the United States: 2009. http://www.census.gov/prod/2010pubs/p60-238.pdf. Accessed: 8/16/2011.
U.S. Geological Survey. "Geology of Colorado". Parameters: Geologic Map, Quaternary Faults, Cities and Towns. Scale 1"=75 miles. Dataset: National Atlas of the United States http://nationalatlas.gov Accessed August 15, 2011.
Subject Archive EntryGenbank accession number, available at: http://www.ncbi.nlm.nih.gov.
DOI stands for Digital Object Identifier and is a unique number used to precisely locate electronic items like webpages, articles, files, etc. A DOI is persistent, which means it does not "break" the way a URL can when a website is updated.