Fingering (4-ply) yarn is the most popular yarn weight for knitters. This is the perfect weight of yarn for socks, excellent for shawls and mitts, but also great for lightweight sweaters and garments.
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.