The Tigers needed a fifth starter in the wake of Phil Coke transitioning back to the bullpen. While moving Coke probably solves the bullpen need Detroit has, it opens up a hole in the starting rotation.
The Tigers picked up starter Doug Fister and reliever David Pauley from the Seattle Mariners for minor-league third baseman Fransisco Martinez , outfielder Casper Wells, as well as starter/reliever Charlie Furbush and a player to be named later.
Fister, 27, has a 3.33 ERA despite a 3-12 record in 21 games, all starts. He has 3 CGs as well.
Pauley, on the other hand, has been shut down in the ‘pen with a 2.15 ERA in 54.1 innings of relief. Pauley had previously been with the Red Sox in ’06 and ’08 as well as Seattle last year and this year.

On the Mariners side of things they pick up AA third baseman Francisco Martinez. Martinez has been rated in the Tigers top ten prospects, but he might have been expendable due to higher rated prospect Nick Castellanos, who also happens to play third. Martinez has raw power and runs well.
Wells is an outfielder who can play all three outfield spots. He has decent power, but most project him as a fourth outfielder. He has been up and down with the Tigers and the Toledo Mud Hens. In 64 games in the big leagues this year he has 4 HRs and 12 RBIs. He also owns a career .286 average in 100 games.
Furbush has appeared mostly out of the bullpen with 15 relief appearances and 2 starts, totaling 32.1 innings pitched.
The Tigers get the starting pitcher they need to fill in their rotation in Fister. Not to mention the fact that he is under team control until 2015 and isn’t arbitration eligible until after next year.  They also get Pauley who will help them in their bullpen.

This is a bit puzzling for the Mariners. They get a young arm in Furbush, but Wells and Martinez don’t make a huge amount of sense for Seattle. Both are good players, don’t get me wrong, but the M’s already have young outfielders galore seeing as they have been using Greg Halman, Make Carp, Carlos Peguero, Michael Saunders and Mike Wilson. To add Wells to this group is good, but somebody is going to have to DH or be benched in AAA because of the number of guys at the position in AAA and the Majors. Martinez should be a really good player, but the Mariners already have Alex Liddi and Matt Mangini and the hot corner in AAA . You can’t slide any of the three to first because of Justin Smoak. They already have the aforementioned outfield depth, so that probably ties up the DH spot as well. It also seems odd to sacrifice a guy who has an ERA of 3.33 who is under team control until 2015.