Someone who's inattentive isn't paying enough attention. An inattentive lunch companion might spend the whole meal texting on his phone.
Inattentive people are sometimes distracted, like an inattentive babysitter who loses track of the kids he's watching at the playground. You can also be inattentive simply through indifference or boredom — if you're inattentive at the opera, you're not paying attention to it because you're not interested in it. Inattentive tacks the "not" prefix in- onto attentive, from the Latin root attendere, "give heed to" or "stretch toward."