" Last year" is a specific past time which means a year before this year. " In the last year" isn't a specific time which means at some point between last year until now. " In the last year" is used in The Present Perfect tense. Example: Simple Past: I went to Hawaii last year. Present Perfect: I have been to Hawaii in the last year.

Last year I ate twenty pounds of chocolate. This could mean anytime last year. In the last year, I ate twenty pounds of chocolate between May and June. Or they can be used in the same way.