This seems like a stupid question, but the closest answer I could find in the sitecore docs I have (online and offline), is that shared fields are "shared across languages". I took that to mean a shared field's content would be the same across all translations of that item in sitecore.
Is that correct?
Often, you'll see shared fields for Image field types. For example, a picture of an apple isn't going to be translated. It makes sense that the apple appears across all language versions, and hence the need to make this image field shared. When in Content Editor, content authors will see the [shared] text next to field title.
Refer to Yan's blog about using Siecore Rocks as this new tool helps clear up the confusion.