Fusion is a process in which the nuclei or two atoms combine to form a new element. In stars, hydrogen atoms fuse to form helium. Many elements are formed in stars as successivley larger atoms fuse. Fission is the breaking down of a nucleus into two or more particles. Since lead has a smaller atomic number than uranium, it has fewer protons so it stands to reason that uranium splits into smaller particles to form lead rather than fusing with other nuclei.
The decay chain, or series of steps in which uranium 238 undergoes fission to become lead, is shown in the diagram. Each alpha decay reduces the mass by 4 and the atomic number by 2. Each beta decay incresases the atomic number by one without changing the mass. This is because the beta particle, a high-energy electron, is formed when a neutron changes into a proton in the atom's nucleus.
Uraniun-238 loses an alpha particle to become Thorium-234, which loses an beta particle to become protactinium-234, which loses a beta particle to become uranium-234, and so forth. As you follow the nuclear decays in the graph you will see the atomic number on the x-axis and the mass number on the y-axis. The final product is lead-206.