Embed HTML widget (mini application) on client's page in IFRAME

I need to expose some pieces of functionality of my system to clients. I've never done that before, so I wonder would it be possible with IFRAME. But before I got down to work, could you advice is it even possible for this use case:

• client's site user opens a page with iframe to my system
• user does some actions (in this case browsers the calendar and schedules visits)
• user checks out. And this is important bit: user needs to sign in / sign up on my page
• after sign in / sign up, user confirms his actions

I wonder is it all possible and if this is good approach with iframe in general (eg. will I run into any obstacles), or I should rather think of somethig else (like popup window with url to my system).

Thanks