Josh Howard is a base year contract player because the Mavs resigned him at a higher salary than his original contract. The Mavs can only count 50% of his new contract (about 4.5 million) when trading him. Since the Mavs are over the salary cap they can only receive back 125% + 100,000 of the outgoing salaries in a trade. So if the Mavs traded Josh Howard straight up, they could only receive a player in return who makes about 5.7 million (125% of 4.5 + 100K) even though Howard's actual salary is 9 million.

Devin Harris is a "poison pill", just like Josh Howard was last year. Any player who signed an extension of their rookie contract that has not yet gone into effect is a poison pill. Dallas can only use Devin's current year salary for outgoing value (about 4 million). The team receiving Harris has to use an incoming salary value that is the average of his current year salary and the annual salary of his extension -- about 8.1 million (45+4 / 5+1).

Josh Howard's BYC clause expires on July 31st 2008. Devin Harris will be a poison pill until next season when he becomes a BYC like Howard this year.

Hopefully that didn't make you more confused. Please ask more questions and I'll try my best.