A Weld refers to a BasePart surface type, or it refers to the RBXL object.
2006-2013: Welds looked like infinite X's.
2013–Present: X's became separated.
A Surface Weld is a surface on a BasePart that makes parts touching that surface connect to the brick with the weld surface. Weld surfaces cannot survive explosions. Surface Welds appear as a pattern of many intersecting lines, resembling "X"s.
A Weld Object is a ROBLOX object that can bind two parts together, regardless if the parts are or aren't touching. To use a weld object "PartA" and "PartB" must be defined.