What is the difference between Mishnah and Gemara?

The Mishnah is the foundation: short laws in Hebrew, divided into six orders. The Gemara is the structure built on it: a long Aramaic discussion that explains and expands the Mishnah.

How to tell them apart on the page

On the Talmud page the Mishnah is marked “Matni'” and the Gemara starts with “Gem'”. Each Mishnah comes before the Gemara that discusses it.