Re: Basic Question about Sequences LONG ANSWER

Well, you kind of have the right idea. It goes like this:

The Avid Basics (skip this if you already know it):

Project-->
In every project you have Bins, which hold effects, sequences, master clips (for Video, Audio, imported graphics, etc...). Essentially, everything in your project goes into a bin, and you can make as many bins as you want (I don't think there's a limit). This helps you organize your footage. You may have one bin for video, one for audio, one for effects, one for titles, one for imported graphics...

Bins-->
These hold your master clips, which are pointers to your media files (which are stored on your media drive in a folder called OMFI MediaFiles if you're dealing with OMF footage, or MXF folders if you're dealing with MXF). More on media files later. Back to bins.
You can also store effects in your bins, so if you have an effect you plan on using often you can make a bin for it and stick it in there. You can mix and match anything you want in a bin--you can put sequences in with audio and titles. Bins are like folders in a filing cabinet and allow you to organize your footage, etc...

Media Files-->
All of your media is stored in an OMFI MediaFiles folder at the root of your media drives (i.e. M:OMFI MediaFiles). Avid uses OMF and MXF format--don't ask me the difference because I don't know. I use OMF--MXF is (I think) for 10Bit files, HD, etc... When you import an avi or quicktime, Avid converts it to an OMF or MXF file and files it away with all your other media. This is nice because you no longer have to worry about the original file being moved somewhere and losing your link (unless you need to reimport it--then you have to go find it if you've moved it. If you haven't moved it Avid will remember where it was). Avid's media management is the best in the business.

Clips-->
The Audio and Video you capture or import into your system. These can also be imported graphics, titles you make within the Avid, etc...

In Answer to Your Question about Sequences:

Sequences-->
A sequence is your edit, basically. You take your clips, your audio, your graphics, your titles and anything else you want and you put it into a sequence. You can make a sequence as long or as short as you want (although I've heard that Avid with Mojo will only play out 3 hours at a time, and Avid with Adrenaline will play 6. I don't, I haven't tested it because I don't have tapes that run that long).
Sequences can consist of up to (I think) 16 tracks of video and audio on an Xpress Pro system, or 24 tracks of audio and video on Media Composer. Symphony and Nitris--I'm not sure. Of course, once you learn how to nest tracks you'll soon realize you have an unlimited number of them, but that discussion is for another day.
How you do your edit is up to you. If you're doing a short form piece or a commercial for broadcast you're looking for anything from :10 seconds to 10 minutes or longer. For short pieces like this I use one sequence for my edit. If I decide to play around with the edit but don't want to lose what I've already done I simply select the sequence and hit Ctrl+D to duplicate it. Now I can play in that one without affecting the original. You can make unlimited sequences, allowing you the freedom to try everything under the sun without affecting your original cut.
For long form pieces (say an hour long short or a 2 hour movie) you might find it beneficial to break each sequence down into a scene. Say this sequence is called Part 1 (how's that for a generic name?)--this may be everything up until the first commercial break (for an TV show). Edit away. When you want to work on the second part, make a new sequence, called Part 2. When you're ready to put it all together into one master sequence, do this:
Take your sequence for Part 1 and drag it from your bin to your Source Monitor (the one on the left). It is now loaded as if it's a clip. Cut it into a new sequence called Master. Drag Part 2 to your source monitor (or Ctrl+Alt+Double Click to open it in it's own monitor) and cut that into your Master Sequence, after Part 1. Continue until everything is in order.
Now some systems allow for nested sequences--Nest Sequence 1 into Sequence 2. Make a change in Sequence 1 and it changes in Sequence 2. Avid doesn't do this. If you make a change to a sequence the change effects that sequence only.

That, in a very large nutshell, is the world's most basic overview of Avid, and how sequences can work for you. Cut everything into one big line, or divide it up by scene and combine them later--the choice is entirely yours. If you have anymore questions, I promise my answer will be shorter and more concise in the future, and if I have mistakes in here other's will help to correct them.

Mike.