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This is an almost impossible answer, because North American signal practice varies wildly by railroad and over time. Here's a link that will take you to recent Union Pacific practice: http://www.lundsten.dk/us_signaling/aspects_up1988/ Note that signals have indications (what you see); names (what they're called), and indications (what you do when you see the indication). The presence or lack of a number plate is significant; on many North American railroads the lack of a number plate indicates the signal is an Absolute, and that signal grants authority to occupy the track it protects. The presence of a number plate indicates the signal is an Automatic Block Signal, which does NOT grant authority and merely indicates the condition of the track beyond it. Absolute Signals, in very general terms, surround and protect control points, interlockings, crossings at grade, crossovers, junction switches, etc. -- anyplace that a dispatcher will want to grant authority to a train or protect a main track. Automatic Block Signals are placed at appropriate braking distances (and where they're visible as possible) at intermediate points between Absolute Signals. Most, but not all, Absolute Signals are controlled signals, that is, a dispatcher or operator can request it give a proceed indication (stop is default). But there are many non-controlled Absolute Signals, such as at crossings at grade with another railroad or drawbridges. These signals respond to track conditions. So why the difference? An Absolute Signal indicating stop cannot be passed without oral or written permission from a dispatcher or operator, whereas Automatic Block Signals are usually stop-and-proceed at restricted speed (half the limit of vision, and not more than 20 mph), and sometimes not even the stop is required. Obviously, you don't want a train doing a stop and proceed at a diamond or drawbridge, but you don't need a dispatcher or operator to control that signal in all cases, either. This is incredibly brief. A broad, general, introductory explanation of common North American signal practice would take about 20,000 words and a lot of diagrams. Trains did a two-part article back in the 1950s that was very good; we're in the process of updating that article so we can run it again, because a lot has changed (and a lot hasn't).
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