OP, I'm assuming that you mean you want to be able to load the same music on all your different devices. That really depends on how/where you bought it.
Anything you buy on iTunes is associated with that iTunes account. Apple does let you load them on multiple devices, however the assumption is that you own each device you are loading it on. For music, it's not a big deal as you would only need to load it once, but for apps, its a much bigger issue as you would need the credentials each time an update is issued.
Anything you buy from
Amazon is tagged to your Amazon account, but as they sell normal MP3s, there really is nothing preventing you from loading them onto multiple iPods, or any other devices that play MP3s.
If you get tracks from any other music store that are in the MP3 format, they will be handled just like the Amazon tracks. You can load them onto whatever you have. The same applies for music you rip from CDs you own.
However, the big catch is that all the devices share the same computer. Apple only lets you associate your device and iTunes account with a set number of computers. So if you log in on your computer, then your kids computer, then your DH computer, you will eventually run out of computers and then they won't let you load content onto them any more. If you try to associate a device with too many computers, it will either wipe the device, or not let you sync the data. So any iTunes content would really need to be stored on a single computer.
Any normal MP3s can be copied between computers and loaded into mulitple iTunes installs without any issues.
So short answer, yes they can share content. iTunes purchased content has restrictions and really needs to be kept on one computer. MP3s from Amazon, ripped from CDs etc can be passed around like candy and loaded onto as many computers as you own.
Legal Note: Depending on your take on the law, there are limits on how many copies of a song you can make, even if you own the CD. So copying content you bought for your use to other devices may technically be illegal under some current laws depending on your jurisdiction. I am not a lawyer, and most people pay as much attention to this as they do to Jaywalking laws, but keep it in mind.
