what are orbitals and what are shells and difference between them?
Within each shell you have orbitals s, p, d, etc There is one s-orbital per shell, three p-orbitals per shell, five d-orbitals per shell and so on. Each orbital can hold up to a pair of electrons.
What the orbital is (at least for A level purposes) is really just a region of space of a certain shape where you are most likely to find the electron(s) it contains. In the case of the s-orbital, that's basically a sphere centred on the nucleus. In the case of the p-orbitals, they look a bit like dumbells aligned along the x-, y- and z-axis with the nucleus at the origin. The shapes of the d and higher orbitals are a bit more involved, but you don't need to know that for A level