The Nexus S is a stock Android phone. That means that it carries no UI layers or "skins," such as HTC Sense, Samsung's TouchWiz, or Motorola's Blur. It is a stock Android experience, and as such is Google's Developer Phone (the most recent prior version was the HTC Nexus One). Many prefer the pure Android experience, although not all do.
Stock Android phones generally receive updates to a new major version of Android earlier than those with a "skin," which is one major advantage, and why these are used as a developer phone. For example, the HTC Nexus One was the first smartphone to gain official access to Froyo, or Android 2.2.
The Samsung Nexus S comes in three variants. They include two GSM models, one for frequencies compatible with AT&T 3G service, and one with frequencies compatible with T-Mobile 3G service. There is also a version of the phone for Sprint's CDMA service, that includes WiMax 4G (and that model is dubbed the Samsung Nexus S 4G).
As defined by their names, neither of the GSM versions is compatible with the (faux) 4G service on AT&T or T-Mobile. All of the versions run Gingerbread (or Android 2.3). Also, all of them include a near-field communication (NFC) chip, meaning they will work without extra hardware with Google's mobile payment service, Google Wallet.
All of them are available, as noted earlier, for free. That free(dom) still comes with a price: they are only available on contract to the associated carriers, meaning you either have to sign a new two-year contract, or else be eligible for an upgrade.
Purchases can be made locally (store locator here) or via the Best Buy website, as well.
Spec-wise, the Nexus S sports a Samsung S5PC110 processor, which a 45 nm 1 GHz ARM Cortex A8-based CPU core with a PowerVR SGX 540 GPU. The CPU core, code-named "Hummingbird", was co-developed by Samsung and Intrinsity. It is not, unfortunately, a dual-core CPU. The GPU was designed by Imagination Technologies.
It also has 512 MB of dedicated RAM (Mobile DDR), which is a little low in the age of smartphones like the Motorola Atrix 4G, which has 1GB of RAM. The device has 16 GB of storage, but as another negative, does not have microSD card support, surprisingly.
The Nexus S also has a 4.0-inch "Contour Display," which is a slightly curved glass touchscreen with a Super AMOLED display (although outside of Canada, the U.S. and the U.K., it has a Super LCD display).
Update: Google is "advertising" the Nexus S sale on its homepage, as well.
Image Source: Wikimedia Commons