Zero-configuration networking tool
Bonjour is a communication protocol that enables discovering devices and services on an IP network. Its main selling point is that it requires no pre-configuration, facilitating the process of finding other devices for the users.
A background manager
Bonjour is a piece of software that you don't use directly as you would other apps on your PC. Instead, it operates silently in the background.
Your PC uses Bonjour to communicate over a local data network and perform various low-complexity, repetitive tasks. By doing so, it enables shared media libraries on iTunes, for example.
If you're a Windows user who doesn't employ any Apple devices, you don't need Bonjour. However, if you are combining gadgets from the two operating systems, you'll find it beneficial.
For example, if you use Apple TV or an iPhone but don't have a Mac, you'll manage them from your Windows PC. However, it would usually require a lot of network setup to do so - Bonjour facilitates it.
Moreover, you'll find that some third-party apps such as Adobe's Creative Suite use Bonjour.
Sometimes, Apple-based apps will automatically install Bonjour. However, note that other services, such as Hamachi and PuTTY, offer better emulation and more efficient networks if you feel like optimizing.
Wired and wireless
Since Bonjour uses the Internet Protocol to run, it runs both wired and wireless. In either case, it uses zero-configuration, automatically locating and keeping track of devices.
This program uses a 'link-local addressing' scheme, assigning IP addresses to local devices. It also provides location services to various apps.
Bonjour doesn't slow down your Internet although it's constantly running. It also supports various versions of Windows OS.
Since this program comes from Apple, it's vital for Mac devices, and a lot of Apple software depends on it.
Highly convenient for some
Although quite old, Bonjour is a highly convenient, free tool to use even today. However, if you're not using any Apple devices or software on your Windows, you won't find it useful.