"Letos" (this year) and "(V)loni" (last year) are Czech adverbs of time. "Tento rok" = "Letošní rok" (this year) and "Minulý rok" = "Loňský rok" (last year) are not adverbs in Czech. These noun (rok) with pronoun (tento) or adjectives (letošní, minulý, loňský) are used as adverbial phrases of time in sentences. The adverb "letos" comes from the adjective "letošní" - they have the same linguistic rout as well as the adverbs "loni" (or "vloni") comes from the adjective "loňský". Both forms have the same meaning, but those adverbs are shorter and better in everyday and less formal Czech speech. The adverbial phrases are longer and seem to be more formal in Czech. Enjoy.

There is no difference between loni / minulý rok and letos / tento rok. Maybe minulý rok is more standard or more used in written language. I do not know where loni comes from but letos come from old Slavonic toto leto - this summer - this year.