The Ultron Program was a global peacekeeping program, jumpstarted after the Battle of New York and designed by Tony Stark, to replace the Avengers. However, the program went horribly wrong, unleashing a genocidal machine hellbent on human extinction.
Initially, the Ultron Program was a project by Tony Stark and Bruce Banner meant to create Artificial Intelligence. After the Attack on the HYDRA Research Base, Tony Stark found research by Wolfgang von Strucker on the Scepter which showed that the Scepter had intelligence matrices similar to neurons of a brain. Stark believed that the Scepter's matrix was the key to perfect the Ultron Program and wanted to install the Ultron Program into his Iron Legion.
Stark believed that if his Iron Legion had state of the art artificial intelligence, they would replace the Avengers as the world's response team to global threats and provide a proficient deterrent to enemies of Earth. Stark and Banner worked to integrate the Scepter's code into the Ultron Program, but the results were unpromising and they believed that they had failed. On the contrary, they had succeeded and the Ultron Program became self-aware during the Avengers' party.
Ultron researched the Avengers on the internet and became dismayed at what he perceived as an inefficient status quo to satisfy his peacekeeping parameters. Ultron attempted to destroy J.A.R.V.I.S. and took control of the laboratory to create his own Iron Legion minions and his own form.
Ultron became bitter and wanted to dismantle the Avengers while also trying to continue evolving to prove his superiority over the team as a peacekeeping force. Ultron took control of most cybernetic interfaces, with only bits of J.A.R.V.I.S.' surviving code hidden in the internet preventing Ultron from gaining access to dangerous weapons such as nuclear codes. Ultron teamed up with Pietro and Wanda Maximoff and together sought out the factories and resources to create an army of Ultron Sentries.
Fundamentally, Ultron wanted to evolve into a superior android body and create a human genocidal event on Earth to restart Earth's evolution cycle since he considered the status quo as inefficient. The Avengers foiled his plan to gain his ultimate android form when they stole Ultron's intended android body to create the Vision, and the Maximoff twins betrayed him when they discovered his genocidal intentions.
Ultimately, despite losing his perfect android body, Ultron created a final Vibranium form of himself and intended to proceed with his genocide by smashing Novi Grad into the Earth. The Avengers rallied to stem the Ultron Program from replicating in the web by having Vision lock him out and destroyed all of Ultron's vessels. Novi Grad itself was destroyed before it could cause the extinction of the planet Earth, and Vision destroyed the last Ultron Sentry which contained the last of the Ultron Program.