Datasheets are instruction manuals for electronic components. They (hopefully) explain exactly what a component does and how to use it. Unfortunately these documents are usually written by engineers for other engineers, and as such they can often be difficult to read, especially for newcomers. Nevertheless, datasheets are still the best place to find the details you need to design a circuit or get one working.
A datasheet’s contents will vary widely depending on the type of part, but they will usually have most of the following sections:
The first page is usually a summary of the part’s function and features. This is where you can quickly find a description of the part's functionality, the basic specifications(numbers that describe what a part needs and can do), and sometimes a functional block diagram that shows the internal functions of the part. This page will often give you a good first impression as to whether potential part will work for your project or not:
A pinout lists the part’s pins, their functions, and where they’re physically located on the part for various packages the part might be available in. Note the special marks on the part for determining where pin 1 is (this is important when you plug the part into your circuit!), and how the pins are numbered (the below parts are numbered counterclockwise). You'll find some acronyms here: VCC is the supply voltage (commonly 5V or 3.3V), CLK is clock, CLR is clear, OE is output enable, etc. These acronyms should be spelled out later in the datasheet, but if not, try Google or Wikipedia. If a pin has a star next to it or a line over the name, that's an indication that the pin is active low which means that you'll pull the pin low (0V) to activate it, rather than H (VCC):
Detailed tables of electrical specifications follow. These will often list the absolute maximum ratings a part can withstand before being damaged. Never exceed these or you'll be replacing a possibly expensive part!
You'll also see the more normal recommended operating conditions. These may include voltage and current ranges for various functions, timing information, temperature ranges, bus addresses, and other useful performance information. The below excerpt contains a good example where the fine print can help you out: "Note 3" in this set of specifications states that "All unused inputs of the device must be held at VCC or GND to ensure proper device operation." This is a reminder to tie all unused inputs H or L to prevent them from "floating" between H and L which can make your circuit malfunction and be difficult to debug:
Some parts will have one or more graphs showing the part’s performance vs. various criteria (supply voltage, temperature, etc.) Keep an eye out for "safe zones" where reliable operation is guaranteed: