DLG

http://www.georgiaspatial.org/?page_id=94

The DLG stands for digital line graph. They contain a wide variety of information including boundaries, roads, utility lines, etc.  This DLG shows railways, airports, road segments, etc.

DRG

http://egsc.usgs.gov/isb/pubs/factsheets/fs08801.html

A digital raster graphic is a scanned image of the U.S. geological survey standard series topographic map.  The image inside the neatline is georeferenced to the surface of the earth and fit to the Universal Transverse Mercator projection.  The color values are standard.  The georeferencing process removes errors caused by stretching and shrinking of materials.  In general, errors are small.

isopleths

http://www.krisweb.com/kristenmile/krisdb/html/krisweb/maps/geomap_ten.htm

These are contour lines that depict a variable which cannot be measured at a point, but which instead must be calculated from date collected over an area. Population density is one example.  This image shows the rainfall isopleths for the Ten Mile River watershed.

isopachs

http://seeps.geol.ucsb.edu/pages/isopach.html

The contours that make up an isopach map display the thickness of a rock unit as apposed to the true veritcal thickness. They are of equal thickness over an area.  This image shows the thickness of overburden on the ocean floor and the increasingly pink colors represent greater sediment thickness.

isohyets

http://www.scoop.co.nz/stories/AK0808/S00090.htm

isohyets are lines drawn on a map connecting points that receive equal amounts of rainfall.  The isohyet contours shown above are showing the month's rainfall in 10mm bands.

isotachs

http://www.crh.noaa.gov/iwx/?n=superoutbreak_meteorology

these are lines on a given surface that connect points with equal wind speed.  They are normally represented on surfaces of constant elevation.  The wind isotachs in the image are the dashed red contours and color fill.

isobar

http://rst.gsfc.nasa.gov/Sect14/Sect14_1c.html

An isobar is an imaginary line or a line on a map or chart connecting or marking places of equal barometric pressure.  This is a map of the U.S. that shows the isobarsar 500 millibars.