They are both similar and are used as the past tense form and past participle of to learn.The difference between them is that "learned" is used in American English. "Learnt" is used in British English as well as English from other countries. There are other similar verbs with this difference, such as: burned, burnt... dreamed, dreamt... spelled, spelt...spilled, spilt. Once again, American vs. British English (with the nt endling).