Over the last few years many answer on here told the person asking the question to check a certain component with a multimeter. This has caused some basic questions. How do I do this? Here is the short description of some of the more common components and how to check those. It will be a basic outline only, not a complete course in electronics.
A capacitor is a small, most of the time, electrical/electronics component on most circuit boards, that can perform various functions. When a capacitor is placed in a circuit with an active current, electrons from the negative side build up on the closest plate(the negative flows to the positive, that is why the negative is the active lead, although many capacitors are not polarized). Once the plate can no longer hold them, they are forced past the dielectric and onto the other plate, thus displacing the electrons back into the circuit. This is called the discharge.Electrical components are very sensitive to voltage swings, and as such a power spike can kill those expensive parts. Capacitors condition DC voltage to other components and thus provide a steady power supply. AC current is rectified by diodes, so instead of AC there are pulses of DC from zero volts to peak. When a capacitor from the power line is connected to ground and the DC will not pass, but as the pulse fills up the cap, it reduces the current flow and the effective voltage. While the feed voltage goes down to zero, the capacitor begins to leak out its contents, this smoothes the output voltage and current. Therefore a capacitor is placed inline to a component, allowing for absorbing of spikes and supplementing valleys, this in turn keeps a constant power supply to the component.
There is a multitude of different types of capacitors, they are often used differently in circuits.The all to familiar round tin can style capacitors are usually electroltyic capacitors. They are made with one or two sheets of metal, separated by a dielectric. The dieletric can be air (simplest capacitor) or other non-conductive materials. The metal plate foils, separated by the dielectric, are then rolled up similar to a fruit roll-up, and placed into the can. These work great for bulk filtering, but they are not very efficient at high frequencies.
Ceramic disk capacitors are great for higher frequencies, but are not good to do bulk filtering because ceramic disk capacitors get to big in size for higher values of capacitance.In circuits where it is vital to keep a voltage source stable, there is usually a large electrolytic capacitor in parallel with a ceramic disk capacitor.The electrolytic will do most of the work, whereas the small ceramic disk capacitor will filter off the high frequency that the big electrolytic capacitor misses.
Then there are tantalum capacitors. These are small, but have a greater capacitance in relation to their size than ceramic disk capacitors. These are more costly, but find plenty of use on the circuit boards of small electronic devices.
This is pretty much just the basics about capacitors, and if one needs or wants to learn more, there are plenty of electronic courses out there. Now the main question here is, how to check a capacitor to see if it needs replacing.
To perform a check on a capacitor while it is still installed in a circuit, an ESR meter will be necessary. If the capacitor is removed from the circuit then a multimeter set as an ohm meter can be used, but only to test an all-or-nothing test. It will only show if the capacitor is completely dead, or not. It will not determine if the capacitor is in good or poor condition. To determine if a capacitor is functioning at the right value (capacitance), a capacitor tester will be necessary. Of course, this also holds true to determine the value of an unknown capacitor.