Welding  is a process that joins materials, usually metals or thermoplastics, by joining them together. This is often done by melting the work piece and adding a filler material to form a pool of molten material that cools to become a strong joint.  Sometime pressure is used in the process along with heat to produce the weld.

 

Soldering and brazing is a different process that involve melting a lower-melting-point material between the work pieces to form a bond between them.  The work pieces themselves are not melted in any way to form the bond.