A tablet does not have a keyboard so typing is done on a virtual keyboard, which is fine for short messages but not for typign a long document.
Not all tablets have USB slots.
Most tablets dont run Windows so you cant install your Windows apps on them. (Microsoft have recently started to bring out some Windows tablets but they are not as common as ipad or Android tablets).
As bluetoffee says, there is some overlap but not a total overlap so they both have their place.
Tablets are really best for just casual internet based activities and not what you may call serious work.