間 The concept of this kanji is similar to English "for". For example 1時 means 1o'clock. 1時間 means 1hour. When we say the first week , we say 第1週. When we say that we stayed in Kyoto for a week, we say 京都に1週間、滞在しました。But 間 has other meanings too.

First, 週 と 週間 の 意味 "は"どう ちがい ます か。sounds natural. 週 represents a unit of week. For example, 二週 means two weeks, and 数週(すうしゅう) means some weeks. 週間 consists 週 and 間. 間 means "during". Therefore, ２週間 means "during two weeks". However, strangely, we often say "２週間の間" In this case, the first "間" means "the time span". So, it means "during a time span of two weeks."

Hi 週間 is used when we want to express the duration of one week or several weeks. ex) 雨が一週間降り続いた　(It has been raining for one week) 二週間後にアメリカに出発する (I'm going to leave for the US after 2 weeks) 週 is used to state 'When', not duration of the time. eg) 今週は雨ばかりだった　(It was raining all the days this week) 私は二週間後にはアメリカにいるでしょう　(I'll be in the US in 2 weeks)