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.

学習is noun, 学ぶ is verb.