The Measure Tool lets you measure the distance between objects, between an object and a page element (e.g., a placed guide, the page center or the page edge) or between guides. This can be done vertically, horizontally or diagonally.
The tool also lets you scale your document or the currently selected artboard by either setting a scale factor (e.g., 1:100, 1mm:1m) or by scaling to a known distance (e.g., 10 meters) on a currently measured object dimension.
The following settings can be adjusted from the context toolbar: