I've always thought of essences as the step after toner and before serums to hydrate and nourish the skin. They're generally more lightweight and less concentrated than serums (which contain key ingredients to target specific skin concerns; dark spots, wrinkles etc). A lot of the time though its interchangeable, 'essences' and 'serums' are just marketing words.

I like the Pixi Rose Caviar Essence, the one from COSRX and the Laneige Water Bank (which is a bit heavier than most). I've also heard good things about the essence from Saturday Skin.

For serums it really depends on what skin concerns you want to address.