Hamas is an Islamist Palestinian political organization and militant group that currently controls Gaza. Hamas has long wanted to destroy Israel and replace it with a Palestinian state. So they are basically fighting over the land. (Gaza was originally controlled by Egypt until 1967, but then Israel occupied it during the Six-Day War.) Hamas has been attacking Israel since Hamas’s founding in 1987.
In the 1900’s and 2000’s, they attacked Israel mostly with suicide bombers but have since used mostly rocket attacks. Although Hamas controls Gaza, it is under an Israeli blockade which makes life very difficult for the people living there. The blockade began when the Israelis feared the inflow of commercial goods into Gaza could be used to make weapons against Israel.