"Solve step", "Solve all" and "Check solution" buttons
The 
button will generate the next step of the solution process.
The 
button will generate the entire solution process. During the solve process, you may click on the "Solve All" button whenever you wish; it will always display all the remaining steps of the process.
The 
button checks whether the generated solution is correct. You are probably not going to use this button very often because, when necessary, the software incorporates the checking into the solution process (i.e. when solving a radical equation).
You can use the "Check Solution" button when checking is not performed automatically (i.e. when solving a linear equation, or after finding the inverse of a matrix).