Thankfully, coffee pods are not complicated. They are basically just small, pre-packaged units of coffee grounds that are used once to brew a single cup of coffee. Using pods eliminates the need to measure out loose grounds, and also means there are no filters to change. It's a very clean and convenient way to dispense your coffee. The most wide-spread form of coffee pod first came out with the Senseo, and it's just a little round paper mesh bag (like a teabag), filled with coffee grounds. That's it. Water is forced through the pod to make coffee, and you usually just use each pod once.
These simple pods that are not machine-specific can be used with the Senseo machines as well as Melitta, Bunn and one of the Krups models. These are the ones that are actually labeled as "pods". There are other makes of coffee machines that are designed to use their own specific format of coffee for their machines. Each kind has it's own name (Nespresso uses capsules, Keurig uses K-cups, Tassimo uses T-discs) but many people just group them all together as coffee pods along with the Senseo types. These other types of pods are a sizable step up in complexity and function. Rather than just a filter paper and coffee combination, they are larger and made of plastic. Each one can hold more ingredients than plain coffee grounds, meaning that you can make other drinks with the same machine.\n\n
In the case of Tassimo, their T-discs have a bar code on the top that tells the machine what kind of drink is being made, eliminating the need for any specific controls. It automatically adjusts temperature, water pressure and timing to suit perfectly. That's a very powerful pod. When they first came out, coffee pods were handy but pretty limited. Today, you can find just about any type of coffee in pods. And that includes dozens of flavors, roasts and styles.
You can also get pods that will make tea, spicy chai and hot chocolate (though not all machines will offer all of these options). Some even come with milk ingredients so that you can use your coffee maker to foam up some milk for an instant cappuccino or latte. The Senseo type pods are not proprietary to any one maker, so you can actually buy them from a number of different manufacturers. Java One, Miss Ellie and Wolfgang Puck all have their own lines of coffee pods for machines that take the generic paper pod.
The biggest advantage of using a pod-based machine is that you quickly brew up one cup at a time, so your coffee is always fresh. Letting it sit and get stale in a carafe is a thing of the past.