The basics of audio plugins 

What is a plugin?
A plugin is a small program or application that works within a host program. The host program is, in this case, your main music program, whether it be Logic, Samplitude, Reason, ProTools, or any other. The reason for adding plugins is to add flexibility and options that the host program doesn’t provide. Most programs come with at least some effects such as reverb, EQ, compression, delay, etc., and some are brilliant and have become standard in the industry, but others come with just bare-bones effects. Whatever the case, most of us have the need or want to expand the palette of effects available to us to create more efficient workflows, more interesting mixes, or to add more instruments to our productions. Plugins are commonly made by third-party companies that are not affiliated with the host programs, but some are add-ons from within the company that created the host program.
There are thousands of plugins available, from incredibly basic to mind-bendingly complex; from free to extremely costly; and from single plugins to suites of dozens of them that come packaged together and must be upgraded periodically to keep pace with technology and the needs of the engineer.
What are the types of plugins?
The most common format of plugins today is the VST (Virtual Studio Technology) plugin. I would estimate that 80% of the host programs out there utilize this format. These plugins work in both Mac and PC formats.
DirectX, or DX, is next, and is used on PCs only. It’s becoming less and less common. The last two, and least common, are RTAS/TDM, created for ProTools, and AU or Audio Unit, which is made for Logic Studio in both Mac and PC formats.
Reason has recently entered the fray with what they call “rack extensions.” These are plugins that can be used only in Reason. Some are effects and some are instruments.
There are also virtual instrument plugins that come in VSTi and DXi formats. These are virtual instruments, most commonly keyboards, that actually produce sounds, not effects. They can be played or triggered by real keyboards connected via MIDI connections or can be played by a mouse. Obviously, the keyboard method gives much, much more flexibility and control than a mouse. These virtual keyboards can give you access to things as basic as a terrible piano sound and as complex as huge banks of multi-layered, editable stacks of synthesizer sounds. For the most part, you get what you pay for, but there are still steals out there. Demo some of them and read reviews. There’s plenty to like for every ear and every style of music.
How do I get them to work?
Generally, all plugins are all installed into the same folder. Your DAW will access this folder whenever you, as the engineer, pull up the plugin. Some plugins have self-installers that just require a few clicks and will find the right folder, or at least let you direct it to the right folder, and some are a little more difficult to figure out. You CAN keep them scattered wherever they happen to install, but then finding them later becomes more difficult and it makes your computer work a little harder when it has to look in several different places for them. The most common place to find/put them is in a folder named VSTPlugins. Other formats can go in there as well. I like to keep all of mine in the same place when they allow it.
Don’t be too alarmed if they’re not recognized right away when you install them or drop them in the folder. It often requires a restart of the program or sometimes even the whole system and perhaps even a re-scan of the plugin folder by the host program to make everything work. And sometimes it might even require a visit to the plugin manufacturer’s website or a call to the help desk.
A Word Of Caution
It’s tempting to think that plugins can fix every mix issue in the world or make a great song out of anything.
Not true.
Plugins, or at least most plugins, should be thought of like the icing on the cake or the spices on a slab of chicken. If that cake or chicken tastes terrible, no amount of the extras are going to make it delicious. They may hide the original flavor somewhat, but that underlying taste is still in there. I hear mixes all the time where people tried to load up on effects to hide bad sounds, and all it sounds like is a more complicated and cloudy bad sound.
Work VERY hard on getting the song to sound solid and right through good performances, good mic technique and good songwriting before you even pull out your bag of plugins, and your need for extra plugins diminishes, or they can then be used a spice to make the songs more interesting to the public, which is what you’re looking for in the end.
The bottom line is this: No one ever danced in a club or walked down the street whistling the sound of a plugin. Make your song great FIRST, then spice it up with the fun stuff.