The Bible contains certain passages that often uses the word 'dragon'. I've always wondered what this means. Are they talking about actual dragons, dinosaurs, serpents, or is 'dragon' used as a metaphor for how evil Satan and demons are?
They are talking about serpents, not dragons or dinosaurs. Dragons were mythical figures and dinosaurs were prehistoric lizards. For the most part serpents are used as metaphors for evil and they are associated with Satan or the Devil, such as in the book of Revelation 12:9. But serpents were also a source of life and healing, as in Numbers 21:6-9, where Moses makes a bonze serpent so that anyone that was beaten by a poisonous snake would look at it and live. This idea is conveyed also in John 3:14-15, where the life giving characteristic of Moses’ serpent is applied to Jesus’ death on the cross.
In the Old Testament, and starting in Genesis 3, the serpent is seen in a negative light, as tempting the first human couple, or as the primordial enemy of God, called Leviathan in Isaiah 27:1 and Rahab in Job 26:12; Psalms 89:11, and Isaiah 51:9. These personifications of the primeval chaos come from Mesopotamia and date back to the time of the exile.