Google frequently updates its algorithm, and sometimes these updates have huge effects on numerous sites. Panda and Penguin are two of the most well-known these days. Google also launches regular data refreshes for these updates.
While even these data refreshes are enough to keep webmasters on their toes, they are much smaller than the updates themselves.
Google's Matt Cutts has talked about the difference between an algorithm update and a data refresh in the past. He put out a blog post all the way back in 2006 on the topic. Given that this was years before Panda and Penguin, it seems worth highlighting now, as businesses continue to struggle with these updates (tip of the hat to Search Engine Journal for linking to this post).
Here are the straight forward definitions Cutts gave:
Algorithm update: Typically yields changes in the search results on the larger end of the spectrum. Algorithms can change at any time, but noticeable changes tend to be less frequent.
Data refresh: When data is refreshed within an existing algorithm. Changes are typically toward the less-impactful end of the spectrum, and are often so small that people don’t even notice.
In that post, Cutts also pointed to a video of himself talking about the differences:
Algorithm updates involve specific signals being tweaked. For instance, PageRank could matter more, or less, Cutts explains in the video. With a data refresh the input to that algorithm is being changed. The data that the algorithm works on is being changed.
He uses a car metaphor, saying that an algorithm update is like changing a part in the car, such as the engine. A data refresh, he says, is more like changing the gas.
Data refreshes happen all the time, he says. PageRank, for example, gets refreshed constantly.
In the end, I'm not sure how much any of this matters to the average webmaster. If your site was hit by an update, or by a data refresh, you probably don't care what the technical name for it is, as long as you can identify the update it's based on, and make the necessary adjustments to gain back your Google traffic.