Yo-yos come in several different shapes. These shapes have a big effect on what style a yo-yo is best for and how it plays.
The Classic or Imperial shape is frequently used for 2A. Its narrow string gap makes it hard to land on the string for string tricks. However, this shape has fallen out of favor with many modern 2A players who prefer the modified shape. Classic yo-yos are sometimes referred to as imperial, slimline or tournament, where tournament is a wider yo-yo shape.
The modified shape has a narrow string gap but not as narrow as a classic shape and is also more rounded. It has become the most popular 2A yo-yo shape and is quite narrow, sometimes narrower than classic. Modified yo-yos are usually good loopers but can also do simple string tricks. Modified shape is also sometimes referred to as Flywheel shape.
The butterfly shape is excellent for string tricks due to a very wide gap but usually loops very poorly. Butterfly shaped yo-yos are favoured in 1A, 3A, 4A (OS) and 5A (FH). They can have either a rounded or 'v-shaped' profile (see pics) and which one to use tends to be a matter of the yo-yoer's preference. Yo-yos intended for OS play are very wide (see pic above) and v-shaped to make the initial throw and catch easier. H-shape is very significant because it has more rim weight than other shapes.