Leaders inspire, heal, and aid the other characters in an adventuring group. Leaders have good defenses, but their strength lies in powers that protect their companions and target specific foes for the party to concentrate on.
Clerics and warlords (and other leaders) encourage and motivate their adventuring companions, but just because they fill the leader role doesn’t mean they’re necessarily a group’s spokesperson or commander. The party leader—if the group has one—might as easily be a charismatic warlock or an authoritative paladin. Leaders (the role) fulfill their function through their mechanics; party leaders are born through roleplaying.