What is Solder Paste?

The fact that it is a paste means that it can be easily applied to the board during PCB assembly.

The solder particles are a mixture of solder. Traditionally this used to be tin and lead, but with RoHS  legislation has been introduced around the world, to only use lead-free solders. These may be made from a variety of mixtures. One is 99.7% tin and 0.3% copper, whereas there are other mixtures that include other metals including tin.