ISBLANK evaluates the cell and returns TRUE when it is completely empty — no content, no formula, nothing — and FALSE when it has any value including text, numbers, dates, or a formula result.
IF uses that TRUE or FALSE to choose which output to return. The alternative A2="" compares the cell to an empty string and returns TRUE for both genuinely empty cells and cells containing a formula that evaluates to "". This distinction matters: a cell with =IF(A1>0,"Yes","") where A1 is zero looks empty but ISBLANK returns FALSE because the cell contains a formula. If you want that cell treated as blank, use A2="" instead of ISBLANK. For the inverse — checking that a cell is not blank — either NOT(ISBLANK(A2)) or A2<>"" works, and both are used as the if-cell-is-not-blank check in workflow formulas. Combining ISBLANK with AND adds a second condition that must also be true; combining with OR triggers the result when any one of several conditions is met.