Tomatillos, or Physalis Philadelphica. are part of the Tomato/Potatoe family and are closely related and have a similar look to Cape Gooseberry (Physalis Peruviana) but aren't as sweet tasting. The plants are grown for their fruits which are covered with a papery husk. Inside is a berry similar to a small (cherry-sized) tomato, which can be eaten raw or made into a salsa (much like tomatoes). The fruit flavour is a mixture of tomato and sweet pepper so goes really well in any dishes that you'd put those in.
Tomatillos grow much like their tomato cousins. They are not frost tolerant so sow in doors in a green house. They tend to germinate slightly sooner than tomatos in our experience though. Once all risk of frost has passed they can be planted out. Or if you have space in your polytunnel or greenhouse then they will grow much quicker and more productively for the extra warmth.
Many people have had a bad experience growing Tomatillos because they feel they are too bitter or tart. But that is usually because they were picked too early. Tomatillos form an outer husk well before the fruit inside. So it can be difficult to tell when they are ready. A good way to know this is if the fruit has swelled to stretch the husk and the husk has dried out and is like a dried leaf skeleton of covering the fruit. Another way to ensure that you get ripe fruits is to wait until they fall to the group and harvest them from there.