USGS topographic maps make use of contour lines to show the shape of the earth's surface. They may also show many other kinds of geographic features, including roads, railroads, rivers, streams, lakes, buildings, built-up areas, boundaries, place or feature names, mountains, elevations, survey control points, vegetation types, and much more.
In order to connect to the USGS Topo service, please follow the steps shown here, using the following URL:
http://basemap.nationalmap.gov/ArcGIS/rest/services/USGSTopo/MapServer/tile/{z}/{y}/{x}