Bhajan is a form of yogic chanting, similar to kirtan. Bhajan chants tend to be more complex, and more poetic, and are therefore more lyrically challenging than kirtan chants, for non-Indian language speakers. Kirtan chants are designed solely to repeat the yogic "names of God", in Sanskrit, where every syllable is beneficial, vibrationally, to our neurophysiology, and our overall yogic evolution.
Please also see the AYP FAQ entries for: Chanting Kirtan