In v7 and earlier, you can although most times it's hard to accomplish what you need because the UI has a lot going on. I've done in before when I was in a pinch but I wouldn't make a habit of it.
There is no APP or other site to go to to manage your site via tablet or smartphone. Version 8 (coming soon!) has some enhancements to the overall design for the editors and should help with tablet and/or smartphone.