Plug-ins extend the functionality of the blog. For example, a plug-in can turn a blog into an online store, or a photo gallery, or a portfolio. Or a plug-in can provide things like an automated backup or spam protection. They usually need
to be uploaded for installation, and can require coding knowledge to
configure and use.
I think that “sidebar bling” is a good description for
widgets – these are easy-to-use code “snipets” that enhance your blog–mostly in terms of offering your readers some more information: about
you, or your blog, or just general information (e.g., Google maps,
visitor’s locations, and recent posts/comments etc.). Widgets can
usually be added to the blog via the control panel without the need
for any coding knowledge, but are limited to use in the sidebars.