The cytoplasm is basically everything in the cell except for the nucleus - a sea of little factories carrying out the jobs they were created to do. The cytoplasm is therefore made up of organelles and something called the cytosol. The cytosol is a liquid that surrounds all the organelles, made up predominantly of water, but also of lots of ions and big molecules.
The concentrations of ions in the cytosol is incredibly important, because it affects what happens at the membrane of the cell - including the resting membrane potential. The concentration of potassium inside a cell is usually about 120mM, whereas outside the cell it is more like 5mM (i.e. the the concentration of potassium inside the cell is about 24 times the concentration of potassium outside).
As well as potassium, sodium is an important ion in the cytosol. Its concentration is much higher outside the cell (about 145mM) than inside (about 10mM). This means that when a sodium channel opens, sodium will flow into the cell - which is how an action potential happens.
Although most of the cytosol is water, with lots of ions dissolved in it, it also has bigger molecules that bulk it out and can have an important effect in organising what goes on in the cytosol.One of the most important features of organelles is that they are wrapped up in membranes to keep them separate from each other. Because each organelle has a specific purpose, it is kept from the others. This prevents it from mixing up two processes, and ensures the cell can function properly.
This article is going to describe a few of the organelles you can find in human cells, but its important to remember that there are loads of different organelles that you could find if you were to pick a cell at random. There are lots of processes which a bacteria might be carrying out which a human cell doesn't need to. Or a plant, for instance, needs to carry out photosynthesis, and has specialised organelles for that which a human simply doesn't...