My wife and I both have an iPhone and a Mac. We also have an Apple TV in our living room. However, sharing our iTunes content (movies, music, books, apps, etc.) between these devices can be tricky, especially since we regularly hot swap our Macs and iPhones for general purposes, depending on whatever device is nearer.
Thankfully, Apple has made this a whole lot easier with Home Sharing, particularly with the latest version in iOS 4.3. Using the same Wi-Fi connection, Home Sharing allows you to share all the media on the iTunes libraries in your home with each other, and with your iOS devices.
Here's how to enable Home Sharing in your home.
First, make sure you have the latest version of iTunes on all the Macs in your home. Second, make sure you have the latest version of iOS on all the iOS devices in your home. Don't forget this includes your Apple TV if you have one.
Once you've done all the updates, go to iTunes and enable Home Sharing by simply clicking on Advanced > Turn on Home Sharing. You'll be prompted to enter your Apple ID (for complete instructions visit this Apple tutorial).
Now, enable Home Sharing on your iPhone, iPod touch or iPad by tapping on Setting > iPod and entering the same Apple ID you used on your Mac under the Home Sharing heading (for complete instructions visit this Apple tutorial).
Make sure you also enable Home Sharing on your Apple TV by using the same Apple ID, too (for complete instructions visit this Apple tutorial).
The main thing to remember here is that you must use the same Apple ID when enabling Home Sharing on all the various devices in your household, otherwise it won't work.