If you're buying through the volume channel, you can select between a number of different programs:
- Open Licence
- Open Value
- Open Value Subscription
- Select/Select Plus
- Enterprise Agreement
- Enterprise Subscription Agreement
Select and Select Plus are very similar. Many people will be on a Select Agreement already, but you can't sign new ones, it's Select Plus only for new ones, so Select will disappear when the last ones expire in five years time.
Open is a standard volume-purchase arrangement; you put in a bulk order (five or more licences) and you get a discount off the single-licence price. If you get software assurance, it runs three years from the date of purchase, and is renewable on the anniversary.
Open Value and Enterprise Agreement are similar to each other, but you need to be a much bigger organisation to sign an Enterprise Agreement (250 desk minimum). In either case you pick a standardised licence for every user from a short list of options (e.g. you can either get Core CAL Suite or Enterprise CAL Suite, but not pick and choose the individual CALs within those suites, nor have some users on one and some on the other), multiply by the total number of users in your whole organisation and then write Microsoft a large cheque each year for three years. You have to get Software Assurance; you can renew after three years and, if you do, you get a discount on the renewal compared to the original purchase.
Open Value Subscription and Enterprise Subscription Agreement are the same as OV/EA, except you pay less per year and you don't own the licences at the end of the agreement. subscriptions cost more than an SA renewal, but less than L+SA. It's a very good deal if your organisation is getting significantly smaller in the near future; some people should look at this in the present climate.
Select and Select Plus are similar to Open Licence, except they are on-going arrangements and all of your orders over a year build up to get you better bulk discount rates, whereas Open only gets you the bulk discount based on the size of the individual order. However, you need to buy a certain number of points (points being a representation of how much you spend) each year to keep your Select or Select Plus contract. Typically, you need 250+ desks for Select, but if you're buying Visual Studio for each user, then you rack up a hell of a lot of points and you wouldn't need anything like as many users. Looking at my agreement, if you have VS Team Suite for every user, then you'd be able to maintain Select with about 35 users.
There are special offers for ISVs and Microsoft Partners, though - so look out for those; I'm only describing end-user licensing.