Maybe you have known the difference, but some people haven't. So, let see the difference.
Tobi is a japanese word, actually mean 'to jump'. In igo, tobi really means 'to jump', since it is used to jump outside (to the center).
The squared stone is example of tobi
Hiraki is similiar to tobi, but there is a difference. Hiraki is also "a jump", but not to outside. It is a jump in the same vertical line or we can call it moving at the side.
The squared stone is example of hiraki
Niken biraki is usually used to make live.
The squared stone is niken biraki example
Igo's term is, sometimes, complicated, but I found it that it's more interesting to use the terms while discussing ;)