'Onto' is used to express movement on a surface and is mostly used with a verb that denotes movement. ... Thedifference between the two would be that 'Into' mostly expresses movement or action with the result that someone or something becomes enclosed or surrounded by something else..
These two are prepositions
into is used with some verbs like settle for example :I just settled myself into the chair.
And for onto : I slipped as I stepped onto the floor