The human body is made up of trillions of cells. Cells have their own lifespan and old cells will die and be replaced by new cells. Cancer describes a problem where the normally ordered process of cell death and replication is not working properly, and excess cells are created resulting in tumors. There are malignant tumors which can spread into other tissues and benign tumors which do not spread into other tissues. Both tumors can be deadly.
Human beings start as one cell (zygote) which divides into 2 cells, and those 2 cells divide into 4 cells, and so on. The adult human has trillions of cells, and most of these cells replicate themselves through a process called mitosis. The phases of mitosis are
Prophase -> Prometaphase -> Metaphase -> Anaphase -> Telophase -> Cytokinesis
The DNA is replicated into a new cell every time it is formed. Sometimes damage is done to the DNA when a cell is replicating it for a new cell, but the cells can also repair this damage by themselves.
Cancer is a general term because there are many different types of cancer. One thing in common between all types is that the cells keep dividing and creating new cells. Normally there is a limit to the amount of cells the human body will create, and once it gets to that limit it will stop, but with cancer the human body keeps creating cells well past the limit which causes tumors.
Cancer cells are not following the rules that normal cells follow which tell them to stop replicating after a certain limit. That is why tumors will form from an excess amount of cells built up in one place. Also, cancer cells can affect the cells around them and convince other parts of your body to help them build up the tumor. Even the immune system can be tricked by cancer cells, because cancer cells are still your own cells.