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As miki 美輝 said, 学習 is a noun. However, you can use it as a verb by adding the verb する ('to do') after it, so 学ぶ = 学習する. Another difference is that 学ぶ is a 和語 (wago - native Japanese word) whereas 学習 is a 漢語 (kango - Sino-Japanese word). Generally speaking, wago are more often used in spoken language and informal texts, while in formal and scientific texts many of them are replaced with kango.
