On and onto are prepositions.
We use on when we refer to a position on a surface (on the table, on the ocean, on the moon, on the roof, on the bus):
Your keys are on the table.
The men were standing on the roof.
We use onto to talk about direction or movement to a position on a surface, usually with a verb that expresses movement:
The cat climbed onto the roof.
She emptied the suitcase full of clothes onto the floor.
We use on to describe a position along a road or river or by the sea or by a lake:
The hotel is on the road opposite the beach.
They have a fabulous house on a lake in Ireland.
We use onto to describe movement towards an end position along a road or river:
The path leads onto the main road.
We use on or onto with very little difference in meaning to refer to attachment or movement of something to something else. Onto gives a stronger feeling of movement:
There’s a battery pack with the camera that you can clip onto a belt.
You can save the data onto your hard disk.
Have you put the pictures on your memory stick?