If you're neglectful, you're not as careful or attentive as you should be. A neglectful babysitter might lose track of where her charges are.
A neglectful cat owner might forget to feed his pet, and a neglectful host might wander away from his party guests to check his email or read a magazine. In either case, the neglectful person is being thoughtless or inconsiderate. The verb neglect, "fail to properly care for," is at the root of neglectful, from the Latin neglegere, "to make light of, disregard, or not trouble oneself about," or literally "not to pick up."