The Difference Between 間 and 間に

The difference between 間 and 間に in Japanese

間 (あいだ) 

Grammar: Nの／Aい／Vている・Vる＋間

Meaning: throughout, continuously…

Usage: Indicates an action that continues throughout a period. The actions in clause 1 and clause 2 are of equal length

Examples:
彼は会議の間ずっと眠りをしていた。
He dozed off throughout the meeting.

一生懸命泳いでいる間はいやなことも忘れてしまう。
While concentrating on swimming, we forget all our frustrations.

間に (あいだに)

Grammar: Nの／Aな　＋な／Aい／Vている・Vる＋間に

Meaning: while, during

Usage :The second clause expresses a quick, brief action. The action in clause 1 is long; the action in clause 2 is short and takes place at a certain point within clause 1 (not as long as clause 1)

Examples:
留守の間に泥棒が入った。
While I was out, a thief sneaked into the house.

家族が皆寝ている間に家を出ることした。
I decided to leave the house while everyone was still asleep.

In short:

「あいだ」 The action takes place throughout the period AB.
「あいだに」 The action takes place at a certain point within the period AB. Usually objective; the speaker is clearly aware of that period.

See also:
Basic Japanese Grammar PDF
9 Japanese Grammar Patterns Involving もの