Quoting from the Wikipedia:

Dyscalculia (or math disability) is a specific learning disability involving innate difficulty in learning or comprehending arithmetic. It is akin to dyslexia and includes difficulty in understanding numbers, learning how to manipulate numbers, learning maths facts, and a number of other related symptoms (although there is no exact form of the disability).

The following newspaper article contains some interesting findings about dyscalculia.

 Noguchi, Sharon (10/5 2011). Don't get math? Researchers home in on the brain's problem. San Jose Mercury News. Retrieved 1/23/2012 from http://www.toledoblade.com/Education/2011/10/17/Don-t-get-math-Researchers-home-in-on-the-brain-s-problem.html.

Quoting from this article:

The findings are so new that there’s no widely accepted way to diagnose what’s known as dyscalculia, nor any set strategies for coping with it — even though 5 percent to 8 percent of the population is thought to suffer from math learning disability. Consider it the mathematical partner to dyslexia, which impairs reading ability.

But while researchers have explored causes of dyslexia and developed strategies for compensating, the study of dyscalculia lags about 30 years behind. As a result, many people remain stymied by math. And math dysfunction is socially accepted.

The article mentions several interesting research results, such as:

• Having poor estimation skills (which can be measured even in kindergarten) is an indicator of dyscalculia.
• Brain imaging studies indicate that children with dyscalculia don’t activate the parietal cortex, which is critical for number processing, in the same way that other children do.
• It might be that many adults claiming that “I hate math” and “I could never do math” can trace the situations to having to cope with dyscalculia in their math education.

Here is an interesting tidbit from the article. A recent survey report from Sylvan Learning Corporation found:

… that about one-third of 400 children surveyed would sacrifice a month of video gaming or going on Facebook if they could never have to do algebra again, and 71 percent of 534 parents surveyed think helping kids with algebra is harder than teaching them to drive.

No information is provided as to whether this or unhappiness with Algebra is in any way related to dyscalculia. However, this data does suggest that algebra is a major challenge to many students.