A colloquial pronoun used to refer to a person (he/she/that guy), often with familiarity or a slightly derogatory tone.
あいつ、また遅刻してきたよ。
That guy showed up late again.
あいつの言い方は失礼だと思う。
I think the way he/she talks is rude.
子供の頃は、よく『あいつ』って呼んでいた。
When I was a child, I often called people 'aitsū' (that guy).
A colloquial pronoun referring to a non-person (that thing/one), used informally and depending on context can be pejorative or familiar.
あいつ、もう壊れて動かない。
That thing's broken and won't work anymore.
こんなに便利な道具があると、あいつが手放せない。
With such a useful tool, I can't be without it.