When learning something new the fastest way is often to compare it to something similar that you already know well. If you are familiar with either the dependency distribution system common in most JVM based systems, Nexus, or the front end JavaScript system bower, the other system should hopefully be a bit more familiar to you after you have read this post.
Both Bower and Nexus has the same common goal. To provide a distribution channel for prepared artifacts. However they go about the task in quite different ways. The simplest way of explaining the differences between the two systems is to compare the two primary operations which they both share; retrieving artifacts and publishing artifacts.
As you can see the nexus retrieval mechanism is straight forwards, only involving the nexus server and the client. In the case of bower three parts are involved in the transaction: the client, the bower registry and a git repository containing the artifact. This is the key in understanding the bower system. A bower registry simply acts as a map between an artifact id and the git repository which contains all versions of the artifact. The bower registry does not contain the version information, that is maintained as git tags in the git repository. This becomes apparent if we compare the publishing procedures:
The two systems are achieving the same goal, but through very different methods. Nexus is a fully fledged file repository; while bower is little more than a git repository registry and some git commands bundled into a client.
In practice this means that the bower system is much more lightweight than nexus and other centralized distribution systems, such as npm. The Bower registry does none of the heavy file transfer or storage. Instead it completely relies on the Git repositories for its distribution. But this requires that the Git repositories are publicly available without access control (or at least available to everyone which has access to the registry). It also means that the artifact intended for distribution is stored with the source code. One last thing to note is that in contrast to many other JavaScript distribution system bower is fully capable of handling both HTML and CSS, a well as JavaScript.
Hopefully the comparison of Nexus and Bower have shed some light on the the unfamiliar system!
Further reading:
http://chariotsolutions.com/blog/post/javascript-apps-bower-or-not-bower/