To form matter, these atoms will connect to other atoms to create molecules, compounds and even more complex structures, just like Legos. But unlike Legos, it takes trillions and trillions of atoms to make even the smallest item. All three forms are just simply states of increasing organization. Take water for example. At cold temperatures, water forms into ice, which is a type of solid.
Solids are extremely organized.
They form precise little rows and all the atoms connect to other atoms. Because they are so close together, the organized structure is easy to see and recognize. However, as the temperature increases, the ice melts and becomes liquid water. In a liquid, the atoms are less organized. The atoms still remain relatively close together but the structure becomes fluid and less rigid.