The foremost difference between a normal slot (left) and a small slot tungsten bead (right) is the hole is very tight (smaller), which means that you can use them on standard and fine hooks and likely will not fit on 1x strong hooks.
The main reason for using tungsten in fly fishing is its incredible density. The name tungsten means heavy stone from the Swedish (tung+sten ) tung, heavy (from Old Norse thungr), and sten, stone (from Old Norse steinn).
It has a density of 19.3 g/cm3 - the same as Gold, but much heavier than Lead at 11.3 g/cm3 and double that of Brass 9.8 g/cm3.
Small Slot Tungsten (right photo) beads allow the bead to be fitted over smaller hooks and are comparable to the weight of a normal slot bead one size larger. Since the sink rate in water depends on the density and profile of the object if you want a fly to sink fast, add tungsten! It is dense enough that you can use a smaller fly and still have it sink to where the fish are holding and feeding.
Don't confuse much cheaper brass beads with these beads, brass beads are only about half the weight for a given size of the bead. Another plus for tungsten is its low toxicity, it does not appear to harm animals if ingested.
Normal Slot (left photo) notes the larger cutaway.