Understanding Explain?

[Bruce D]

I'm trying to understand MySQL better. I can't seem to figure out why the
second query takes 2 minutes and the first query takes 12 seconds. It
should be noted that the table, KBM, has 250 million records. I'm using
MySQL 5.0.2.
I guess I'm trying to get a better understanding of what "using index" means
in the "extra" column.

Here's the result of the explain on my first query:
SQL-query: explain Select count(1) as counter from kbm force index
(NewEstimatedIncome,State) where 1=1 and ( (State = 'OH') )

id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE kbm ref State,NewEstimatedIncome State 2 const 7958721 Using where,
Using index
Here's the result of the explain on my second query:
SQL-query: explain Select count(1) as counter from kbm force index
(NewEstimatedIncome,State) where 1=1 and ( (State = 'OH') ) and (
(NewEstimatedIncome >= '030' and NewEstimatedIncome <= '039') );

id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE kbm ref State,NewEstimatedIncome State 2 const 7958721 Using where

Any information is appreciated.
Anywhere I can read more about this stuff...besides mysql.com?

TIA
-bruce duncan