关于 means "about" in

Chinese. The word which is put after "关于" is related to the second sentence.

• 关于这个问题我们下次再说。 In this sentence, the subject we talked about next time has something to do with "this question".
• 关于这次实践活动，你最好写个简单的总结吧。 In this sentence, the summary you write is relevant to "the practical activities".

根据 means "according to" ''based on''. The matter which is put after "根据" is the basis of the second sectence.

• 你可以根据年龄对他们进行分类。 In this sentence, ''age'' is the basis of the classification.
• 根据惯例，公司每个人每年都有两个星期的假期。 In this sentence, "tradition" is the basis of the holiday.

Furthermore, 根据 is a noun sometimes, and means basis.

• 他说的话没有事实根据。