Cancer is a word with a wildly potent connotation. As a patient, when we hear the word we immediately assume the worst. Despite "cancer" being a diagnosis of significance, the spectrum of disease is very large, with a vast scope of possible outcomes. 

Cancer is a disease caused by a malfunction of a cell. Cells are the tiny building blocks of our body, like little pieces of Lego. Most other illnesses reflect a problem with a large structure, like an organ. This microscopic level of disease makes cancer difficult to understand - I hope we can simplify it. 

Picture a cell as a little circle filled with jelly. There are lots of different types. Some cells are bone cells, some are liver cells, some are heart cells...you get the point. The thing that determines a cell's type is the nucleus - a bundle of DNA, floating in the middle of the jelly. 
The DNA blueprint in each cell is the same, but certain external messages determine which parts of the DNA is switched on or off. The parts that can be switched on and off are called genes, and they are the template for producing particular proteins. 

Every second of everyday, our cells are dying. To replace the dead cells, a cell nearby will make a copy of itself. It does so by copying it's own DNA, and then splitting in two - this is called 'mitosis'. Our cells are in a constant state of dying and replacement - in a very balanced way. We should only replace what we lose, generally speaking (except in childhood, etc). 

Oncogenes are the genes that tell the cell to grow and divide. 
Tumour suppressor genes are the the genes that tell the cell to stop growing, and sometimes even trigger cell death if necessary. 
It is the balance of these two forces that keeps the cell division process regulated. 

Cancer happens when a spontaneous mutation of one of these regulatory genes occurs. The malignant cell grows and divides at an inappropriate rate. And because the genes that usually trigger cell death are silenced, it grows unchecked. When the cell has divided itself into a large cluster of malignant cells, we call this a malignant tumour. Cancerous tumours are very sneaky, and usually recruit a large number of blood vessels - increasing the likelihood the a mutated cell can break away and move somewhere else in the body. That cell finds a new home and the unregulated growth cycle repeats itself. 

A benign tumour can occur in a similar manner - the key difference is that the mutation causing a benign growth will not result in spread of the cells elsewhere. They stay in the region that they develop, and usually stop growing at a certain point or grow very slowly. 

But why does my DNA mutate in the first place? 

The truth is, cell division is such a complicated process that mistakes happen all the time. We all have cells in our body with incorrect DNA right now. If the cell doesn't have the ability to put an end to itself - we have a back up security system in place, the immune system.

Our immune system is constantly sweeping our cells, making sure everything is in order. If these little guys spot a mutated cell - they will kill it. Unfortunately, cancer occurs when the mutation that happens not only corrupts the cell growth cycle, but also disguises itself from the immune system. That way, it can grow unchecked. 
This is why a lot of modern treatments involve immunotherapy - attempting to harness the power of the body's own defence. 

We will discuss specific treatments (chemotherapy/radiotherapy/surgical) elsewhere - but essentially, they are all trying to put a stop to cancer cells unregulated growth.