This article is about the patriarch. You may be looking for the tribe.
Reuben was the firstborn son of Jacob and Leah, being the oldest of all his brothers and half-brothers (born of Rachel). As an adult, Reuben felt responsible for Joseph's life and saved him from death when he was sold into slavery, later feeling guilt for the incident. While slated to Jacob's heir by birthright, Reuben lost his birthright as a consequence of sleeping with his father's concubine, Bilhah. Little else is known about him other than that he was the progenitor and namesake of the Tribe of Reuben.
Early in her marriage to Jacob, Leah conceived and Reuben was born. She named him Reuben, meaning "Behold a son", in joy realizing that this son was a gift from God. Having been married to Jacob deceptively, Leah was hated by Jacob in favor of her younger sister, Rachel. Therefore, when God blessed her with a son she believed that Jacob's affections would begin to lean towards her.
Early in his life (likely after the birth of Simeon and Levi) Reuben was able to help in the wheat harvest. While picking, he discovered some mandrakes-- plants thought at the time to increase fertility-- and brought them to his mother. When Rachel saw that Leah had mandrakes from Reuben, she demanded them in exchange for the bed of Jacob for the night.
Sleeping with Bilhah
One of the events Reuben is most known for is his sexual encounter with Bilhah. Though Genesis only mentions the event in passing, it is referenced as the reason for Reuben losing the traditional rights of the firstborn.
After burying Rachel near Bethlehem, Jacob lived for a time "beyond Migdal Eder" (probably "the tower of the Flock"). During Jacob's time in Midgal Eder, Reuben went and slept with Bilhah. Regardless of if Reuben was living with Jacob when he did this, Jacob heard of Reuben's act. Though Jacob's immediate response to Reuben's defilement is not given, he later harshly dealt with him when giving his last instructions. Reuben's act would later come back to bite him and his inheritance.
Saving Joseph's Life
Among all of his brothers and step-brothers, Joseph was the favored child. Genesis depicts the brothers collective envy and eventual hatred towards Joseph after he shared his visions of ruling over them.
Reuben was with all of his brothers, except for Joseph, herding his father's flocks at Shechem. After moving the herds to Dothan, Reuben and his brothers were met by Joseph. As he approached them in the distance, the brothers conspired to kill him. Reuben, however, protested his brothers plans. Wanting to save Joseph's life while appeasing his brothers' hatred, Reuben suggested that Joseph be thrown into a cistern. His hope was to return later for Joseph and bring him back to their father. 
After Joseph was cast into a cistern, Reuben later returned to rescue his brother. To his dismay, Joseph had disappeared- he had been sold into slavery to a caravan of Ishamelites/Midianites and was en route to Egypt. Upon this startling discovery, Reuben ripped his clothes in despair.
Remorse for Joseph's Disaster
Years later, Reuben still felt a sense of guilt about Joseph's disappearance and his presumed death. When the sons of Jacob went to Egypt to buy food during the famine, the Egyptian official (who unknown to them was Joseph) demanded the youngest brother, Benjamin, accompany them on a return trip- a test of loyalty. Not realizing that Joseph could understand his language, Reuben exclaimed that this request was the deserved judgement for Joseph's presumed death years ago. Reuben absolved himself, placing the blame on his brothers for bringing judgement upon his entire family.
When the brothers returned home and gave Jacob the request, Jacob was shocked. Jacob had already lost Simeon as collateral to the Egyptians and Joseph was presumed dead- taking Benjamin to Egypt would surely risk his very life. Perhaps out of guilt for this request, Reuben offered the life of both of his sons if anything were to happen to Benjamin.
The Displaced Heir
By the time he died, Reuben had an additional two sons- his four sons being Hanoch, Pallu, Hezron and Camri.
At the end of Jacob's life, at the age of 147, he gave inheritance blessings to all his sons. Reuben, being his first-born was considered first. Jacob remembered the shameful act that Reuben had slept with Bilhah. While Reuben was noted to be strong and the first sign of Jacob's virility, he was also called "unstable as water". Reuben's greatest qualities of strength and honor were reversed, no longer to be celebrated, when he slept with Bilhah on his father's marriage bed- making him turbulent like water. While the specific consequences of this curse is not made immediately clear, the language clearly shows that Reuben's blessings were made void by his dishonorable sexual act.