Think of organs found in a human. They have important functions, such as making substances for converting ammonia to less toxic substances, secreting substances that aid in digestion etc...
Well, that's pretty much how it is with organelles in a cell. The organelles can have similar functions or completely different functions. Some are involved in making proteins, some are responsible for getting rid of toxic substances that got into the cell, others (like in plant cells) are responsible for photosynthesis etc....
So, even though they're smaller and they can/do have different structures and functions, they're like organs to a cell.
Some organelles found in cells are ribosomes, mitochondria, golgi bodies, and the ER.