No, I'm highlighting the ambiguity
The FBI's official definitions of foreign and domestic terrorism each consist of three points, the first two of which are pretty much identical, dealing with violent acts with the purpose of intimidating a civilian population to influence the actions of its government.
They differ on the third point, which defines the geographical scope of each:
"International terrorism" means activities with the following three characteristics:
...
Occur primarily outside the territorial jurisdiction of the U.S., or transcend national boundaries in terms of the means by which they are accomplished, the persons they appear intended to intimidate or coerce, or the locale in which their perpetrators operate or seek asylum.
"Domestic terrorism" means activities with the following three characteristics:
...
Occur primarily within the territorial jurisdiction of the U.S.
The attack in San Bernardino was definitely domestic terrorism, based not just on the passive within-the-US criteria for domestic terrorism, but because it didn't "transcend national boundaries" to take place here at the instigation of a foreign power required to be foreign terrorism.
ISIS has made official public statements in the last day praising the attackers and specifically calling them "supporters". Words matter very much in propaganda: In past attacks for which ISIS claimed credit, they called the perpetrators "soldiers" or "knights". ISIS denies that Farook and Malik were working for them; but they're grateful nonetheless.
I don't think legalistic points alone are going to soothe your fear, but you did ask me to clarify. I still would return to what I keep saying about being brave and not succumbing to panic. Take a deep breath, and remember that you're still more likely to be struck by lightning than to be a victim of a terrorist attack.
And never, ever, forget that we're in the middle of a bitterly contested Presidential election, and the bullshit is flowing freely. The more emotions are attached to an issue in the public sphere, the more skeptical we all need to be.