Tomatillos look very much like what their name implies: mini tomatoes. Depending on the variety, they can even taste like tomatoes.
Varieties of Tomatillos
Tomatillos come in a few different varieties that are different colors when ripe. The varieties that are purple and red when ripe tend to be most like our standard tomatoes, in that they are slightly sweet (they also look more like tomatoes). The varieties that are green or yellow when ripe, tend to be more tart. Those varieties are commonly used in Mexican and Central-American green sauces, while the sweeter varieties tend to be used more like fruit, including making jams and preserves.
Paleo and Auto-Immune protocol
Like tomatoes, tomatillos are also a nightshade, which are known to cause digestive issues in some people. If you’re not sure the effect nightshades have on you, the best way to find out is to eliminate all nightshades from your diet for a while (including tomatillos), then slowly reintroduce them and note how they make you feel.
Mark Sisson has a whole post on the effects nightshades on different people.