I'll give my best mini-recap -- I'm sure others will offer more. I thought there was a sticky, but I don't see it (so for sure, someone will post the link after I type this)!
DVC is a time share. It is a way to pre-pay for your Disney (and other places to some extent) vacations for many years into the future.
For a sum of money up front and annual maintenance fees, you will purchase a number of points in the Disney vacation club. Those points will allow you to have resort accomodations. Unlike some timeshares, you don't buy a certain week at a certain resort. Your points are flexible in that you can use them at any of the DVC resorts and a number of other Disney and non-Disney resorts (those have a service charge though and some extra "rules"). Depending on which resort you choose, what time of year you go, and how big of a room you need -- you will use varying numbers of your points.
You can use up to 3 years of points at one time, by banking and borrowing from the years prior to and after your current year.
You purchase into a "home" DVC resort which gives you a 4 month booking advantage for THAT resort. This can be important for a lot of reasons, but particularly if you plan to travel during peak DVC times (which are not always the same as the peak WDW resort seasons).
For a family of 5 (that's a tricky one -- can you leave one at home?) -- only Animal Kingdom Lodge "officially" allows 5 people in a 1 BR. There are various reports that Member Services will allow 5 in other 1 BRs, but I'm not going down that path! So aside from that, it sounds like you would need a 2 BR which sleeps 8.
It gets you lodging only and some dining/merchandise discounts (one of the best is discounted Annual Passes). No discounts on theme park tickets.
In my opinion, it's a good fit for people who regularly vacation at WDW and plan to continue that (at least every other year) and usually stay in either a deluxe (or possibly a moderate) resort. If you normally stay in a value, it probably won't make sense financially.
This described us -- frequent deluxe visitors, with plans to keep going. Given that, it was kind of a no-brainer for us and it will probably save us money eventually. We are very happy with our purchase (2008) and are planning our 2nd DVC trip for 2008 -- our 3rd Disney trip for 2008.
There's a ton more to be said, but I think I've done my share!
Vicki