It mostly depends the context. Glad is usually used when something bad or worrysome is averted. Ex. "I'm glad he didn't forget his wallet" or "I'm glad I wasn't late for the bus."

The meaning is similar, but 'happy' is stronger than 'glad' ( or in other words 'happier'). We often use 'I'm glad...' as a response, for example, if someone tells you some good news, you might say 'I'm (very/really/so) glad to hear that'. Glad in this sense means pleased/relieved.

I dnt think they both are different. I use them alternately :p