The San Juan Islands are a cluster of islands (surprise) in northwestern Puget Sound, about 80 miles north of Seattle. There are about 172 islands in the group; the four largest - San Juan, Lopez, Orcas, and Shaw - are accessible via the Washington State Ferry terminal in Anacortes. Waldron, Stuart, Blakely, and Decatur Islands are also inhabited by significant numbers of people, but are only reachable by private watercraft or by air.
The earliest inhabitants of the islands were Native Americans, mostly of the Lummi nation. British and Spanish seafarers discovered the islands in the 18th century, but the first European settlers didn't arrive until 1850 or so. Some of these were British and some American, which led to conflicts between the two nations. These nearly exploded into war in 1859, when an American settler on San Juan Island killed a British pig that was rooting through his garbage. US troops confronted British authorities sent to arrest the settler; British warships arrived off San Juan, and war seemed imminent. Fortunately, cooler heads prevailed, and most of the soldiers were withdrawn from the islands; the incident was known afterwards as the Pig War in honor of its only casualty. Since then, the inhabitants of the islands have been mainly farmers, fishermen, and seafarers, with a smattering of smugglers and rum-runners thrown in for good measure. Lately, as the popularity of the area has grown, the population has shifted towards vacationers and retirees (mainly very wealthy ones, which is not necessarily a good development.)
The islands are the remnants of an older continent being subsumed by the North American mainland, and are often quite hilly. Their stone is very old and tends to be faulted and crumbly, leading to the area's very distinctive craggy, rough-hewn look. Beaches are mainly rocky, with a few large sandy areas. The northern part of Puget Sound is in the rain shadow of the Olympic peninsula (covered in rain forest), so the islands tend to be quite dry. They're mostly covered in evergreen pine and fir forests, with sparse underbrush. The overall effect is wonderfully beautiful - dark, crumbling rock spotted with the yellows of lichens and dry grass, with verdant forests carpeting the hills and valleys above.
The weather is generally good. Summer highs are in the mid-70s (although there's always a cool breeze off the water); temperatures usually hover in the 40s and 50s during the winter, with infrequent snowfall on the higher elevations. Winter storms from the northeast can make things a little exciting at times, with high winds and freezing temperatures. With the choppy seas slate grey and the dark grey-green islands on the horizon, even the winter vistas here are beautiful.
There are a number of settlements in the Islands, but only one incorporated town - Friday Harbor, on San Juan Island. There are a growing number of tourist industries operating in Friday Harbor (and the populous areas on Orcas and Lopez,) although the town still retains a great deal of charm. The University of Washington maintains a world-class marine laboratory in the area, as well. Populations on the islands are limited by the lack of available fresh water - their small size and low rainfall makes the availability non-brackish water a major concern.
Wildlife is abundant in the islands. The forests and shores are home to bald eagles and great blue herons. Deer and raccoons live inland; there are no large predators (although a bear once swam to San Juan from the mainland). Seals and otters are a common site in the waters, and dolphins and orcas (killer whales) frequent the area. The increase in water traffic has driven many of the orcas into the more open areas of the sound, but some are still occasionally seen in the channels between the islands.
There's a really excellent writeup on the Pig War by prole - it's well worth the read.