Creating a mobile version of iTunes and matching it to the iPod touch and iPhone was a no-brainer on Apple's part. Both devices have the multimedia capabilities and substantial processing power that modern media files require. Just as important, they have WiFi networking capabilities, and the iPhone can connect via a regular cell signal, too.
When you use iTunes on the iPhone, you can find and download ringtones, music, movies, music videos, TV shows and audiobooks. You can download podcasts, too. Once you find media you'd like to buy, you can immediately begin transferring files to your iPhone. In the case of podcasts, you can transfer the files in their entirety or stream and play them simultaneously.
Download speeds will vary depending on the quality of your connection, but you should know that by default, the iPhone uses a cellular signal for file transfer. If you prefer a faster Wi-Fi transfer, you'll have to manually change this setting.
If you already had an iTunes account on your computer, your iPhone will automatically use the default settings from that account. If you're only interested in podcasts, you don't even need to register for an iTunes account.