What is pellets

Pellets it's - granulated biofuel produced from peat, wood or agricultural waste - standard size cylindrical granulas. Pellets are an environmentally friendly fuel with ash content not exceeding 3%, burning of wood pellets produces exactly as much CO2 as was absorbed by plants during growth, so there is no any danger to Earth opposite to traditional gas fuel. 

Technology of pellet production 
Raw material (usually wood waste) placed to the shredder, where it is crushed to anguish form. The resulting mass foes to the dryer, and then - to the press granulator, where wood flour will be compressed into pellets. Compression with hi pressure increases the temperature of the material, lignin contained in wood softens and bonds the particles into dense cylinders. On the production of one ton of pellets takes 4.5 cubic meters of wood waste.

As pellets are made of waste, there is no need for cutting woods - so there is no danger to forests too. 

What is solid fuel

Solid fuel refers to various types of solid material that are used as fuel to produce energy and provide heating, usually released through combustion (wikiPedia).
Modern solid fuels include charcoal (and other fossil fuel like peat, coal), Hexamine fuel tablets, wood and made of wood fuel pellets, corn, wheat, rye and other grain materials, especially which are used for biofuel production. 

Solid fuels give more heat than gas, they are not explosive and not toxic - this means ecologically clean; solid fuels are used in boilers, ovens and stoves, central heating systems.