Read Committed and Repetable Read

Good article on it in StackOverflow

1) The default isolation level of Read Committed means that Dirty Reads will not occur (although phantom or non-repeatable reads might). It does not guarantee that single rows are locked.

A non-repeatable read can occur in the following situation:

1. Transaction 1 begins2. Transaction 1 read a row3. Transaction 2 begins4. Transaction 2 changes the value of the same row read by Transaction 1
5. Transaction 2 commits6. Transaction 1 reads the row again. Transaction 1 has inconsistent data.

2) Repeatable Read isolation level means the above situation cannot occur (though phantom reads still might). A phantom read can occur in the following situation:

1. Transaction 1 begins2. Transaction 1 read a row3. Transaction 2 begins4. Transaction 2 deletes the row read by Transaction 1
5. Transaction 2 commits. Transaction 1 can no longer repeat its initial read, 
   since the row no longer exists.

If you want to guarantee that data does not chnage while you read it, you would need the Serializable isolation level. I would strongly advise agianst not using the Serializable isolation level unless you absolutely have to, as concurrency will suffer.