Something historic has a great importance to human history. Something historical is related to the past. People with big egos get them mixed up if they say they had a historic family background. Unless they helped win a war, it was probably just historical.
Historic describes something huge that happened in the past, something that affected human history. Historic things can be important figures, like Abraham Lincoln, or pivotal events, like the invention of the printing press. Historic lows are the lowest in history. Here are some historic examples from the news:
The last one closed in October as the museum moves historic objects out of its west wing for a major renovation beginning early next year. (Washington Post)
A private funeral for the rap legend will be held at the historic Grace Baptist Church in Mount Vernon, just north of New York City. (Seattle Times)
Germans have little incentive to save, with interest rates at historic lows. (Business Week)
Something historical, like an event or an object, doesn't have to concern all of human history or be of worldwide importance, it's just something connected to the past, like these examples:
Other blow-by-blow historical Twitter efforts have run aground. (New York Times)
Kugel regularly furnished historical information that other Madoff employees used to fabricate trades, the government charged. (Washington Post)
Our package also included photos, video, story, historical timeline, readers' memories and polls. (Seattle Times)
The Battle for Bunker Hill was an important battle in the Revolutionary War, making it historic. While the battle for the last piece of your mom's chocolate cake might live in your family history as huge event, it's historical.
And by the way, if you pronounce the h- in either historic or historical, you should use a rather than an before it.