A diagram of a sugar molecule
found in outer space, from NASA
Organic molecules means the kind of molecules that living things are made out of - at least, all the living things we know about. All living things, both plants and animals, are made out of molecules that have carbon in them. Because carbon has only four out of eight spaces filled in its valence shell, it's very easy for carbon to link to other atoms (in cluding other carbon atoms). So a lot of big molecules are made mainly out of carbon.
Hydrocarbons are a common kind of organic molecule which have both carbon and hydrogen in them. Hydrocarbons are very common kinds of molecules, because both hydrogen and carbon are very common elements in the Universe. Lots of different kinds of hydrocarbon molecules formed out in space even before the planets formed - we know that sugar, alcohol, and other hydrocarbons are still out there in modern nebulae. The molecule in the diagram here, a kind of sugar, has oxygen atoms in it as well as carbon and hydrogen - sugar is one of a group of hydrocarbons called carbohydrates.
A model of glycine, the simplest amino acid
These simple hydrocarbon molecules sometimes got together in space to form bigger molecules called amino acids. Amino acids use four kinds of atoms - hydrogen, carbon, oxygen, and nitrogen (or sometimes sulphur). We know that amino acids, too, are still out there in modern nebulae.
Once the amino acids got to Earth, around 4.5 billion years ago, some of them eventually joined up in long strings of amino acids and became even bigger molecules called proteins. There were definitely proteins on Earth before four billion years ago. These proteins are the building blocks of living cells, which formed about four billion years ago.