Jupiter was the supreme god of the Romans, and the Lord of the Heavens and the Sky.
Being the lord of the skies he was responsible for all the weather, especially thunder and lightning, he was sometimes referred to as the Thunderer for this reason.
At first Jupiter may have been seen as an agricultural god. As time passed and the Romans became more civilised, he developed and became the god of all human endeavours.
As the greatest of the gods he was also seen as a great champion, and was invoked for victory in all things, especially battle.
Jupiter gave birth to the goddess Minerva from the top of his head, it was Juno’s (Jupiter’s wife) jealousy of this event that was the main reason for the birth of Mars. Mars was born with no input (so to speak) from Jupiter, and was soon to rival Jupiter as the god of war.
He was invoked for luck, wealth, victory, good weather and most human affairs.