what's the difference between transgender and transsexual?

It's not stupid. The problem is that the words are used differently by different people. The basic explanation is that "transsexual" refers to people whose physical bodies don't match their internal sense of gender identity, and "transgender" could be an overarching term encompassing transsexuality, crossdressing, genderqueer, etc. or it could mean the same thing as "transsexual". Some trans folks don't like how "transsexual" sounds like a sexual orientation, so they find "transgender" a preferable word. Then there are some who only use "transsexual" to refer to people who have had gender reassignment surgery--I'm not fond of this definition.

I personally use "transgender" to mean those with incongruous gender identity and biological sex, but I'm aware that the term is flexible.

Transgender is a state of mind, it's when your emotional and psychological gender doesnt match your physical gender. A man trapped in a womans body or a woman trapped in a mans body is how it is referred to sometimes.

Transsexual is physically doing something about being transgendered such as dressing as the opposite sex and/or taking hormones or surgery to 'become' the opposite sex. Basically it's living as and trying to pass as the opposite sex of your given physical gender in the ways mentioned above.