=IF(A1<5,"",IF(AND(A1>=5,A1<10),"A",IF(AND(A1>=10,A1<15),"B",IF(AND(A1>=15,A
1<20),"C","D"))))
would be an example.
It would be useful to point out that this is a bad example as the evaluation
of the IF function is progressive, so
=if(A1<5,"",if(A1<10,"A",if(A1<15,"B",if(A1<20,"C","D"))))
would be sufficient.
Also, note that you can not nest IF function deeper than 7 levels.
A work around is shown at Chip Pearson's site
http://www.cpearson.com/excel/nested.htm
But usually, if this is a limitation, another method is more appropriate
such as building a lookup table and using Vlookup.