In helping folks out in the forums, I find that a lot of people don't have a full grasp of how our sync model works and why content is automatically added (or deleted) from their devices. I also find that users tend to get frustrated due to the fact that there is automatic update of content on their device, but they don't know how or why; or even further, how to control it. In today's topic, I want to enlighten you to the world of sync groups, how they work, how they govern the content on your device, and how you can use them to your advantage and ultimately have a better sync experience.
Sync groups, a kinda-sorta-technical explanation...
A sync group in Zune software is a set of rules that governs / defines a set of media that will be sync'd to a Zune device. You can use sync groups to combine different combinations of music, video, pictures, podcasts & friends in your collection for eventual syncing to your Zune device. There are two different ways to create a Sync group in Zune software, (1) drag and drop a container or containers to the device icon (2) Manually create a sync group and specify the rules for media to be included using the Sync Group editor.
Once a sync group is created it will automatically add / remove content from itself when the player is active. This is achieved by applying the rules of the sync group against the media contained within the user's collection; basically the rules contained within the sync group are based upon various metadata contained within each media file that is in the users collection.
Ok, so what does that mean?
First of all, sync groups are only created when you are in "let me choose" sync mode for a particular content type. So, if you have music set in "let me choose" sync mode, anytime you drag and drop (or right-click and choose "sync with <yourzunename>") any genre, artist, album, song, or playlist (videos, pics, friends, and podcasts, too) to your device, a sync group is created to remember what you sync'd to your device so it stays on the device. You can also create complex sync groups using the sync group editor. It's kind of like autoplaylists specifically made for syncing content to your device. I'll give an example of that later.
With simple sync groups, if I drag and drop the artist Radiohead to my device, a sync group will be created for Radiohead. Once this group is created (see below for example), the software basically says "ok great, anytime my user adds more Radiohead tracks to the collection, I'll put them on their device automatically for them." Accordingly, anytime I add more of Radiohead's music to my library, it will automatically be sync'd to my device. Conversely, if you delete the Radiohead sync group, the content will be removed from your device. Further, if you delete some or all of the Radiohead songs on your computer, they will be deleted from the device. This is a pain point for some users as some folks want to have content that is not on their computer still remain on their device. We have definitely heard this complaint, and while I can't give away the road map, all I can say is that we are listening. At least now you understand why content would be deleted from your device if you delete from your computer; it's all about the sync group...

That's great, but what if I don't want sync groups?
Well, then you are just silly. Kidding of course, but seriously, you don't have to go by the sync group model if you don't want. If you have a larger device (or a smaller collection) then "sync all" mode may be best for you. Sync all mode does just what it says, it syncs all of the content in your collection for that content type. So, if you select sync all mode for pictures, it will sync all of the pictures in your collection to your device. Similar to sync groups though, it will automatically add content to your device as it is added to your collection, and will delete content from your device if it is deleted from your collection.
What if I don't want you to automatically add and delete content from my device?
Now that's just crazy talk! Kidding again, and we realize there are some of you that really want full control of the content that is added and removed from your device. To achieve this, you can connect your device in "guest mode" which will not create any sync groups when you sync content to your device, and hence it won't add or delete content automatically. To do this, disconnect your device. Then go to Settings > Device > Sync Options. Then click the "forget this device" button. Then reconnect your device and select the option to connect as a guest. Now you can sync content to your device, delete it from your machine, and it won't be removed from your device.

Advanced sync group action...
A feature that we added in the 2.5 release is the ability to create complex sync groups based on specific criteria that you choose. These are really like autoplaylists for sync'ing specific content your device. You can create complex sync groups from music, video, and pictures in your collection, and then filter the groups using a multitude of filters. Just like simple sync groups, as content is added to your collection that fits the criteria of the sync group, it is automatically added to your device. To create a complex sync group, navigate to Setttings > Device > Sync Groups, then click the "new group" button. A dialog will launch where you can name your sync group, choose from the source (music, video, or pics), and then filter the group to your liking. I'll outline one of my favorite sync groups below.
Complex sync group example: Favorite TV series...
Here's my scenario: I am a huge fan of "The Office." Needless to say I was stoked when we got the deal with NBC and started offering episodes of The Office for purchase in the Zune Marketplace. I'm continuously buying episodes, so I wanted a way for them to sync automatically. Here's how I do it: I purchase an episode of The Office from Marketplace, then I navigate to Settings > Device > Sync Groups, and click "New Group". I name it "The Office" and set my Video collection as the source. I then choose "The Office" from the TV series dropdown menu, and then click OK. Now the group is created, and the episode I purchased is automatically added to the group and sync'd to my device. Now every time I purchase a new episode of The Office it is automatically sync'd to my device. (Tip: You can even get more creative by using the "Year" filter to create separate sync groups for each season of the TV series).

I encourage you to experiment with the sync group editor and see what cool groups you can come up with. If you do, I think you'll find that you have much more control over the content that syncs to your device, and in turn, a better sync experience. Feel free to add some comments to this post with your favorite sync groups to share with other users. It's fun to see what people come up with, and I might even add a few more of my faves in there for you as well. :) Enjoy!
(for full versions of the example screenshots (and another favorite sync group of mine) click here)