What it is and where it comes from
Kosher gelatin is gelatin approved by a Jewish kosher authority. Many kosher gelatins are made from fish or from cattle slaughtered by Jewish law. But kosher authorities do not all agree on gelatin, and some have accepted gelatin from other sources.
Where you find it
Kosher sweets, marshmallows, desserts and capsules.
What to look for on the label
If the label says kosher fish gelatin, it is halal. For kosher beef gelatin, see beef gelatin: some scholars accept kosher slaughter (Quran 5:5), halal certifiers do not. If the source is not stated, ask.
This is a guide, not a fatwa. Recipes change, so check the label, look for halal certification, and ask your local imam where scholars differ.