Pods are tea-bag-like, single-serving-sized paper sacks of ground, blended espresso intended to fit into special filters provided with many pump espresso machines. They are typically packaged in single-serving foil envelopes. You open the envelope, pop the pod into the filter, and brew. The pod comes out of the filter as cleanly and simply as it went in.
Pods offer clear reassurance for the beginner because they are easy to use and guarantee a proper dose of properly ground coffee. However, they are very expensive, offer limited choice of coffee, and can only be used with pod-compatible pump machines. Furthermore, they create a good deal more waste packaging than whole-bean coffees.
Some pods are proprietary in format and designed to work with only one brand of machine, and visa-versa. Increasingly, however, home pump machines are designed to give users the option of using either regular ground coffee or generic pods that work with any machine that follows something called the ESE (Easing Serving Espresso) standard. I strongly recommend that those interested in brewing with pods opt for maximum flexibility by buying one of the pod-optional, ESE generic format machines rather than a machine that uses a proprietary pod format. Pod-users almost always graduate to regular coffee when two points become apparent to them: first, brewing espresso is not so hard after all, and second, by using pods they are paying too much for less-than-fresh coffee.
Pods, like canned espresso coffees, are almost always blends. Like canned blends, pod blends usually come in a range of styles, from mild northern Italian styles to dark, pungent blends suitable for large milk drinks like caffe latte. Pods offer slightly more choice in coffee than do canned blends, but again, not nearly as much choice as whole bean.