I would call it a financial product designed to ensure that its salesmen can send their kids to college and afford an excellent retirement. It offers the illusion of protection at a high cost. I'll use Thevin's numbers to illustrate how the cap would have affected one's returns over the last 20 years.
The first column is the S&P return for each year, including dividends. The second is the return without dividends, just the index itself. Last, we cap the return at 12% maximum, but 0% minimum. Over the 20 years, the S&P returned $65620 on a $10K investment. (Take off 1% or $656 for a low cost ETF). When we remove dividends we get just $44830 which is almost the same as the hypothetical IUL returned, $45130.
You can debate the numbers all day long, but in the end, you trade a significant portion of your potential return for the feeling of 'safety'. You slept well for 20 years and laughed at the negative years, the crash of 2008, etc, but in the end, my $65620 can withstand a 30% drop as it continues to grow ahead of that $45K. The IUL will lag the S&P by 2% as I've shown, and every instance of "saving you from a crash or negative year" will be offset by the 20%+ return years that are clipped down to 12%. You can see 4 zeroes over 20 years, but also 11 years the returns were capped at 12%.
The data is easily imported from MoneyChimp, and you can see for yourself for any time period back to 1871.
Edit - I was thinking about the longer term, and ran the number for the 30 year period 1985-2014. These were the returns for the extra decade:
You can see, 5 years of 12% capped returns, 2 zeros, one of which occurs in a year that was actually positive for the rest of us, in 1994, even though the index was down, the return with dividends was positive.
And the 30 year summary:
The IUL returned $90,797 on the investor's $10K, vs the $255,051 the S&P indexer saw. Thevin's answer states "This is one of many crediting strategies, but this is one that is most popular and easy to understand. It allows you to particate in the market gains, without the risk of losing your money in a market downturn." From my experience, this crediting is one of the more favorable. I've seen IULs return even less, on average over the years. To the extent one would have been better off in CDs or bonds over the same period.
On a final note - when analyzing a UIL, make sure you understand the crediting rules. Take the time to back test to see how it would have performed over time vs a simple S&P ETF or S&P/bond mix.