These are alternative forms of the past tense and past participle of the verb learn. Both are acceptable, but learned is often used in both British English and American English, while learnt is much more common in British English than in American English.
We learned the news at about three o'clock. They learnt the train times by heart.
There are a number of other verbs which follow the same pattern in forming the past tense and past participle:
I burned/burnt the toast by mistake. He dreamed/dreamt about his holiday. Luke kneeled/knelt down to find his contact lens. Tanya spoiled/spoilt her dinner. She spelled/spelt her surname an unusual way.
Leap, lean, spill, and others are also verbs of this type. They are all irregular verbs, and this is a part of their irregularity.
Learned (but not learnt) is also an adjective, pronounced as two syllables ((ˈlərn|əd)) rather than the one syllable verb (ləːnt or ləːnd). The adjective, when said of a person, means 'having acquired much knowledge through study'. It can also be used of objects, meaning 'showing, requiring, or characterised by learning; scholarly'.
She is a learned and respected teacher. I read the report in an extremely learned journal.
Resource: Oxford Dictionary
