Por.Trailblazers???
Apparently something to do with the settlers who moved west, "blazing new trails" to the Pacific Northwest.
Min.Timberwolfs
Selected by the community, I believe based on the actual animal that inhabits an area in Minnesota
Lakers?
Originally based in Minneapolis, Minnesota, "the land of 10000 lakes"
Spurs
To quote the NBA Encyclopedia, it represents "the Western heritage of Texas".
Cavaliers
Selected by the community, because it represented fearlessness men who never surrendered.
Clippers
Throwback to the San Diego days, named after the sailing ships that passed through San Diego bay.
Mavrics
As with the Spurs, it ties in with the cowboy image of Texas.
Bucks
A quote from the then-GM of the Bucks John Erickson - "the predominance of bucks (in Wisconsin) led us to the name".
Nets (and Mets form baseball)
Mets, I don't know. Nets, called so because that's one of the most important aspects of basketball - the net. Or so the NBA Encyclopedia says.
pistons
Originally the Fort Wayne Zollner Pistons - they were named the Pistons after the owner's (Fred Zollner) company - a piston-making plant. When they moved to Detroit, the "Motor City", the name was still appropriate.
celtics
Chosen because it dates back to the Original Celtics from New York, a barnstorming team with an impressive record. Also due to the Irish heritage in Boston.
Most of that is taken from the NBA Encyclopedia. I believe NBA.com's history section has similar info.