Pena is just 24, but has been in pro ball since he was 16 and in the majors for parts of the last 4 seasons. You can see his stats here. In 302 games he's struck out 288 times while compiling only a .302 OBP. He does have power, having connected for 19 HR last season in 99 games. In the short-term, Pena becomes a fourth outfielder; he's played nearly as many games in center (97) as in right (116). It also gives the Red Sox some insurance against an injury to Manny or David Ortiz by adding another power hitter. But by 2007 he could be starting in RF if Nixon is not re-signed.
The bottom line is that the Red Sox have more pitching prospects than power hitting prospects, so trading a pitcher for some young power made sense. But if the pitching staff falls apart by July and Pena is piling up the strikeouts, it could be a long year for the newest member of the Red Sox.