"onto" vs "on to". What is the difference?

Onto is a preposition meaning "on top of", "to a position on", or "upon".

• Kaley climbed onto the tree limb, dangling precariously over the stream.

It can also mean "fully aware of" or "informed about".

• George realized Amelia was onto the surprise party he was planning.

Use on to as separate words when "on" is part of the verb:

• Alesha caught on to his scheme from the start.
• I'm going to log on to the program tonight.