Difference between 如果 and 要是?
In my opinion, they mean the same:&quot;if&quot;. &quot;要是&quot; is a little informal and more common in oral speaking, while &quot;如果&quot; is formal, I think, but you will see both of them in books.
I could only catch up with one differences in use between them. For &quot;要是“, because there is a &quot;是&quot; inside, when the verb in the &quot;if-clause&quot; is &quot;是(to be)&quot;, &quot;是&quot; can be ommitted, however, &quot;是&quot; can't be ommitted when using &quot;如果”.
Some examples:

要是我，[我]就不会那么做。 If [it were] me, [I] won't do that way.
如果是我，[我]就不会那么做。If [it were] me, [I] won't do that way.

Also, I don't agree with Tom Au that only &quot;要是&quot; has a &quot;wish&quot; mood. I think &quot;如果&quot; can be used in this case too.

要是我有100万就好了。 It's good if I have 1 million,really means I wish I could have 1 million
如果我有100万就好了。 It's good if I have 1 million,really means I wish I could have 1 million