In my opinion, they mean the same:"if". "要是" is a little informal and more common in oral speaking, while "如果" 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 "要是“, because there is a "是" inside, when the verb in the "if-clause" is "是(to be)", "是" can be ommitted, however, "是" can't be ommitted when using "如果”.
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 "要是" has a "wish" mood. I think "如果" 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