Sport Weight (5-ply) is a relatively new gauge category for UK knitters and crocheters, but our American cousins will be more familiar with it.
In a nutshell, it's the gauge between Fingering (4-ply) and DK (double knitting). And you'll sometimes see it described as a "heavy fingering" or "light DK" weight yarn. Usually happiest on 3 - 3.75mm needles with a gauge of approx 23-26 stitches per 10cm, when used on its own.
We think it's the perfect weight for knitted garments. Not too light, not too heavy. But you can also use it for accessories and socks.
Chuck in another yarn, and get a whole new gauge and experience.
You can add other yarns to this weight class to create thicker gauges, and use the yarns in combination to achieve your perfect fabric (in terms of drape, warmth, colour, stitch definition and gauge!).
Don't hesitate to get in touch if you'd like some recommendations. Mixing your yarns is a great way of getting exactly what you want, and a unique result.