I have been hearing a lot about Seattle in the news lately, but I don't know much about the city.  Is Seattle the capital of Washington state?  
No I believe Olympia is Washington's capital. Seattle is however the largest city in the state of Washington. 
What is the population of Seattle?  I assume several million.
Estimated 713,700 residents. That also makes it the largest city in the Pacific Northwest region of North America.
That's amazing.  Is there anything significant either historically or practically about Seattle?
States and remained in the top 5 in May 2015 with an annual growth rate of 2.1%. In July 2016, Seattle was again the fastest-growing major U.S. city, with a 3.1% annual growth rate. Seattle is the northernmost large city in the United States.
The city is situated on an isthmus between Puget Sound (an inlet of the Pacific Ocean) and Lake Washington, about south of the Canada–United States border. A major gateway for trade with Asia, Seattle is the fourth-largest
in the state with a population of over 100,000 (Seattle, Spokane, Tacoma, Vancouver, Bellevue, and Everett) as well as the state capital, Olympia.
Interstate 90 (I-90) is the longest primary Interstate Highway in Washington, which connects Seattle to Spokane and the Idaho border, measuring , while I-82, connecting the Oregon border city of Umatilla to Elensburg via Yakima, is the shortest at . The longest auxiliary Interstate Highway in Washington is I-405, a bypass of Seattle through the Eastside, at , and