Example sentences
nǐ你xué学xí习shén什me么？
What do you study?
tā他xué学xí习hěn很hǎo好。
He studies well.
About this word
学 is to learn and 习 originally meant to practice, from the image of a young bird repeating its wingbeats until flight comes. So 学习 is "learn and practice", covering both halves of the job: taking something in, then drilling it. That second half explains why 习 also turns up in 复习 (review) and 练习 (practice). The word works as both verb and noun: 我学习汉语, 学习很重要. When you mean review specifically, 复习 is the precise choice.
The story behind the character
学
The traditional 學 is a whole scene: 爻 (counting rods for divination) on top, two hands fiddling with them, and 子 (a child) below. A kid working the rods with his hands. That's the oldest picture of learning. Xu Shen wrote 学, 觉悟也: "to awaken."
习
习 is 羽, feathers, sitting on top of 白. Xu Shen explained 習 as 數飛也, flying again and again. The picture is a young bird working its wings, flapping until it gets it right. Practice by repetition is the original sense. 学习 is study plus that kind of drilling.