How do plugins working in the background?

I have a question. Let's say I have 30 plugins that all do a specific thing on a command. Do these 30 plugins constantly run in the background, using up RAM, or is it only used when a command is executed?

 

They are always there, loaded and waiting for being used. But it depends very much on what the plugin is doing and how it is doing it whether they use up a noticeable amount of your ram or not:
Plugins which only execute some code when being triggered are probably not using any of your cpu in the time while no command is executed, and only a few kb of ram (depending on the plugins size).
If the plugin starts some tasks or listens to certain events, those also use your cpu every once and a while / everytime the event is triggered.
If the plugin stores data (like playerdata, or data about regions/zones, or cached chunk processing data (orebfuscator, dynmap, etc)) they probably use some more ram. But still: it varies between each plugin.

 

I don't really want to make a thread so I'm going to ask here. Do you know which plugins cause the most lag? I just got CoreProtect. Would that cause lag?