There are normal blocks, which can be destroyed by one hit, reinforced blocks which require two hits, and super reinforced blocks which require three hits.
There are invincible blocks which cannot be destroyed. These are not required to be destroyed for the level to end since they cannot be destroyed.
There are blocks that eat balls. These blocks look like holes. They can be destroyed by lasers or other balls if you have more than one. They are not required to be destroyed for the level to end since destroying them requires random powerups or losing extra balls.