Myasthenia Gravis is when antibodies attach to the post-synaptic receptor, preventing ACh from binding.
Eaton-Lambert acts on the pre-synaptic area, prevents Calcium influx which then prevents the RELEASE of ACh. It can occur secondary to certain cancers.
MG worsens with repeated movements, whereas Eaton-Lambert gets better with repeated movements.
There is my ghetto summary from memory.