Similarly with the chicken and the egg, there is often confusion surrounding the shaker and the maraca.
A shaker is ultimately a receptacle charged with spherules of stuff. There are a number of variations to choose from and this is where things start to get quite exciting. For example, you can get a shaker the size of an egg (often called egg shakers) which fit comfortably in most sized hands. Then there are shakers that are the size of a tube of BBQ Pringles.
The receptacle can be made of anything, but wood, plastic and metal are the standards. The contents can also be made out of pretty much anything the shaker creator fancies shoving in. All of these variables make a monumental different to the sound of one’s shake, so please make sure to be very careful as to which shaker you opt for. As well as this, different weights, shapes and sizes will result in adapting which technique is best suited for the shaker in hand.
Maracas are basically a different breed of shaker, but with handles and typically come in pairs. There’s nothing stopping you from just playing one, and some people actually prefer the lighter sound of an un-paired pair. In saying that, there is nothing stopping you from doing anything (within reason). If you want to buy that pair of trouser that you window-shopped two weeks ago, then you go for it. They’ll look great. Or they won't.