"Crude" is certainly an interesting word to apply to an explosive that is capable of ripping flesh from limb, and so I wanted to find out what experts mean they say they an explosive device is not sophisticated.
The answer is two-fold: electronics and chemicals. First, a "crude" bomb is likely to use a timing device — a so-called "command" bomb that does not require an outside stimulus, such as a radio frequency, or pressure, or a laser, to detonate. Secondly, it probably does not contain high-grade explosives, the type that you'd find in, say, improvised explosive devices in warzones. If you've ever seen an IED explode, you'll note that the fireball is much bigger than the one we've all seen in Boston. For hundreds of years, gun powder, or black powder, was used as the main ingredient in bombs, and it remains the easiest to procure and then figure out how to detonate.
How can even a small amount of explosives be so dangerous?
It contains a lot of potential energy that can be very rapidly liberated. Even low-grade explosives can deflagrate — which means that the chemical energy travels more quickly than the percussive force, as fast as several hundred meters per second.
Someone standing within, say, 20 feet of a small device can have his leg torn off by shrapnel from the container of the device. But more likely, the traumatic amputations we saw were the result of the shock wave itself, which can slam people to the ground, or into railings, or into buildings, which such force that But many injuries are percussive; the shock wave can compress chest cavities, blow apart blood vessels and snap necks. (Blast lung is the most common cause of death from explosives; the shock wave over pressurizes body cavities, causing ruptures).