Jethro, also called Reuel, was a priest of Midian and father-in-law to Moses. He is mentioned several times throughout Scripture, only directly in the Book of Exodus (while referred to indirectly when his son is mentioned). Jethro fathered seven daughters and owned a flock. One of his daughters, Zipporah was given to Moses, who at the time was a refugee in Midian. Reuel later was an advisor to Moses while he lead the newly formed Nation of Israel and counseled him on government structure.
Reuel was born in Midian and was given his name, which means "friend of God". He would later become a priest of God. It is not known whether or not the priesthood was hereditary in Midian (thus his name) or if he became a priest on his own accords. Regardless it seems the nation of Midian worshipped God and Reuel would have been raised in Yahwism.
At one point Reuel acquired a flock of some sort. He also had seven daughters and one son. He tasked his seven daughters to care and feeding of his flock.
A Son in law
Once Reuel had his flock and his daughters were tending it, Moses came up to Midian as a refugee. Moses rested by the well that his daughter's usually gathered water to feed his flock. When local shepherds began harassing the daughters Moses drove them off and watered Reuel's flock.
With their work done the daughters returned to their father. Reuel noticed they had returned from flock tending much earlier than usual. Reuel's daughters told him of the Egyptian who completed their work. After Midian's priest heard this he told his daughters to give hospitality and invite the man to eat with them. Once Reuel ate with Moses he offered him residence. Moses agreed and in time Reuel gave his daughter Zipporah to him in marriage. Based on Zipporah being wed first she may have been the first born daughter. 
Reuel's son-in-law stayed with him forty years and bore him two grandsons during this time, Gershom and Eliezer. During the forty years Moses tended Reuel's flocks instead of or alongside of his daughters.
After forty years Moses asked Jethro's permission to return to Egypt and permission was granted. Once Moses liberated the Hebrews from enslavement he sent back his wife and son to Jethro who cared for them some time.
Advisor to Moses
After Israel had defeated the Amalekites in battle Jethro heard of Moses's success. So Jethro went to go see his son-in-law, sent word ahead and brought his daughter and grandchildren along with him. He may have brought his son Hobab, explaining his involvement in Israel. After he greeted Moses at Mount Horeb, Moses told him all everything that he had had experienced. Jethro rejoiced at the news of the liberation from Egypt and exclaimed that he now knew that God was greater than all others. This expression may have been just praise, but this could also indicate Jethro was doubting or beginning to worship other gods.
Then Jethro brought a burnt offering, sacrificed it and then ate at a banquet with the elders of Israel. The following day Jethro noticed Moses spending the entire day judging cases. Astonished Jethro asked Moses why he was the sole judge of Israel. When he was told that only Moses knew the laws Jethro began to advise Moses. Jethro suggested that all of Israel should be educated in the laws and taught how to live, something unseen in very few nations at the time. Jethro also told Moses to assign judges over different jurisdictions to more efficiently judge cases. Only the most undisputed cases would be judged by Moses after going through various other judges. Moses listened to his father's advice and after this Jethro was sent home.
The process of judge selection may have taken several weeks to months, so Jethro could have been an advisor for a long period of time.
While Jethro is minor in his textual descriptions he made several impacts on Israel and on Moses.
The name Jethro
While he is referred to as "Jethro" Scripturally this was likely not an actual name of his. The name Reuel which means "friend of God" is used less frequently than Jethro which means excellency. Since the name Reuel was a theonym and he lived in a nation that likely followed God it can be reasoned this was his name. While the name Jethro is used more frequently than Reuel it was always used in the context of him being Moses's father-in-law. Similarly Reuel is used only in reference to him prior to the text stating Moses's marriage to Zipporah (the exception to this is when Hobab is mentioned he is called "the son of Reuel".
Based on all of this it can be reasoned that Jethro was an epithet or a title of respect. Moses would have used the term "Excellency" to refer to his father-in-law out of respect. The name Reuel is used only before this is revealed, so Jethro may have been avoided before the reader knew he was Moses's father-in-law.