Fred's version is excellent, and here's something I wrote up for a friend:
II has two methods of trading:
Deposit first, which allows you to trade from the date of deposit until your use week + 2 years (therefore allowing you up to 3-4 years to use the week).
Request first, which allows you to "hold back" your week unless you receive your desired trade. Starwood allows this for SVN members, and fixed week non-SVN owners, but does not allow this for floating week non-SVN members (the legality of which is extremely questionable).
Different Ownership Types:
Normal Fixed Week Timeshare (including a non-SVN Starwood owner) -- Owner can deposit the week up to 1-2 years in advance (resort sets the timing rule). Some resorts require the owner to prepay an estimated maintenance fee, some do not.
Normal Floating Week Timeshare -- Owner can book week 1 year in advance (with some variances, depending on the resort) and then immediately deposit that week into II. Again, some resorts require the owner to prepay an estimated maintenance fee, some do not. Marriott uses this method -- hence, a knowledgeable Marriott owner wanting to secure a "good trade," will book the best week possible (President's Week, Easter, 4th of July, etc.) for deposit. Other Marriott owners get shut out of popular weeks, even when calling "on the opening" one year in advance. They're very unhappy that exchangers are obtaining weeks they couldn't obtain.
SVN Fixed/Floating Week Timeshare -- Attempting to avoid the Marriott unhappiness, Starwood saves the best weeks for owners who wish to use the resort and has the right to deposit anything it wants when an owner calls to make a deposit to II. Example: Owner books 4th of July, intending to use it ... then something comes up. Owner decides to deposit to II, but Starwood will cancel the 4th of July resv and assign the depositor a mud week in Colorado. This is a good deal for those who wish to use the resort, and a bad deal for those who wish to trade. I know you're all thinking, I bought to use so this is a good thing! Check back in 20 years. As resorts age, and newer, exciting destinations come aboard with other providers, more and more owners want to trade ... hence the almost daily availability of Disney weeks in II -- those people bought to use too (back in 1994) ... but now they'd like to try WSJ, Harborside, Marriott Marbella, Marriott Phuket ... and they'd like a "fair deposit" to have an even shot for a trade. A negotiated blended (generic) trading power with II would solve this problem (Hyatt appears to have this feature, as does Disney and a couple of other part week/part point resort networks).
Non-SVN Floating Week Timeshare -- I don't believe Starwood has the right to deposit an alternative week into II, but they try to do it anyway. They do not appear to deposit an alternative resort, however. Again, a bad deal for exchangers.
Trading Power:
II sets its own rules for trading power, and it doesn't always match StarOption distribution. For example, a prime summer week in the desert (silver season) may have more trading power than a shoulder platinum week simply because more people travel during the summer. This makes it possible to trade a "low StarOption" week for a "high StarOption" week through II. A sort of "timeshare arbitrage opportunity!" Of course, the opposite can also be true if an owner gets stuck with one of Starwood's "bad" deposits, i.e. a Maui owner who gets assigned a mud week in Colorado will find only trade-downs.
Preference Period:
Only Starwood owners are able to exchange into other Starwood units for the first 3 days following a deposit. Marriott owners have a 24-day preference period.
Bulk Deposits:
Starwood uses historical patterns to forecast usage vs. exchanged weeks and "bulk banks" many many weeks with II about 8-12 months in advance. These are immediately available for exchange, despite the lack of an associated owner deposit. When an owner calls to deposit a week, they assign him/her one of these previously deposited weeks. In other words, one can call in August 2007 and be assigned a March 2007 deposit week. This shortens the expiration cycle (the deposit would expire in March 2009 instead of the 2 years added to a "future date" deposit), but the owner generally gets the benefit of an early deposit so it's a wash. It's important to note that this bulk banking process is very tied into the assignment of a week from an alternative resort. Starwood's goal is to "balance the books" of real deposits vs. anticipated deposits, and hence will assign the mud week to the Maui owner (it wasn't fair, but hey ... we're in balance!). In the interest of fairness, the bulk banking process does make it possible for owners who call really late in the year to salvage something out of their ownership. Example, John wakes up on 12-15 and realizes he never did anything with this year's Maui week. No way he'll get a resv for week 51 or 52. But, whatever he's assigned at II, it's better than no usage at all.
Trading Costs:
Domestic: $139
International: $154
Membership: ~$79 - $89 annual (included in SVN membership)