Snip from the article:
Burst Balloons are used when someone is screaming their dialogue. They tend to be more irregular and chaotic than the radio balloon, perhaps with a heavier stroke. Burst balloons typically aren’t italicized, but are often bold with certain words enlarged or underlined for even more emphasis. A less punchy variation on the burst balloon is a regular balloon with a small burst where the tail meets the balloon.
[via Daring Fireball]