Most of the pieces I make are first fused and then slumped.
Fusing means heating the glass in the kiln to the point where the surface becomes softened and sticky. Two pieces lying on top of one another will then start to meld together, and when cooled, become effectively one piece of glass. The higher the temperature used, the more fully melded the individual pieces become, until in a full fuse, it's impossible to feel where one piece ends and another begins. I normally use a relief fuse, in which the edges of individual pieces can still be seen, and can be used, for example, to suggest the bone and muscle structure of an animal.
Slumping means heating the glass to the point where it gets soft enough to bend and/or stretch over or into a mould under its own weight, so creating a 3-dimensional shape. This usually involves a lower temperature than fusing, so is done in a second firing, after the separate pieces have first been fused together to form a flat shape.
The images here illustrate the stages of making a fused and slumped leaf vessel.
"The heat transformation of glass solids is about softening, stretching and flowing as the temperature rises"