The actual expression would look like =IF((Z3=Z4),"=",0) You're telling the cell in which the formula is written to compare Z3 &Z4 and if they're equal, to place an equals sign in the cell.
The syntax is =IF ( logical_test, value_if_true, value_if_false ).
You can muck around with the placing of the formula in order to get it to display correctly. Have a look at this for a decent tutorial. What I do with new formulae is save a copy of my spreadsheet & experiment on that copy until I've got it right, then go from there.
IF Function - Excel 2010 IF Function Quick Tutorial