two different types of technology.
Back-UPS is whats called an "offline" UPS - it means that its not doing anything much until theres a powercut - when the mains fails, a relay closes, the rectifier based battery charger reverses to become a basic squarewave inverter that creates a pseudo-sinewave output running from battery power to supply your computer.
cheap battery backup.
minimal protection from spikes & power transmitted RFI.
no protection from brownouts or surges
50-100ms break in power during the switch between mains and battery
Smart-UPS uses "line-interactive" technology - it still means there is a relay in the circuit, so you still get a small break in power supply, but a Smart-UPS also incorporates a transformer that gives additional filtering for mains interference such as spikes & RFI, and it also can give boost/reduce by automatically tapping or removing another coil on the transformer to reduce the impact of brownouts or surges. They also give a sinewave output rather than squarewave.
Smart-UPS is by far the better choice of the two, but if you want full protection, you need to check out APC's "online" type UPS. More expensive, but they synthetically regenerate the mains sinewave giving you total isolation.