Mercy is compassionate treatment, while justice is the administration of law. Justice may not necessary include mercy.
Mercy is natural. Portia says that the "quality of mercy is not strained", it is not a forced effort but something that one already possesses. Mercy cannot be forced by anyone; it is something that one must come up within himself. Like how "gentle rain" cannot be created artificially, it is sincere.
Mercy also benefits the merciful. Portia says that "earth power doth then show likest God's when mercy seasons justice", implying that man can only become like God when he is merciful.
Mercy is something that is powerful. Portia says, "scepter[s] shows the force of temporal power...but mercy is above this sceptered sway", symbolizing that mercy is more powerful than mere symbols of earthly power, i.e. the crown and the scepter.
Mercy is forgiving. Portia points out that God is merciful, and forgives us for our sins, and "in the course of justice none of us should see salvation".
Only with the mercy of God would they be delivered.
Mercy is reciprocal, and "twice blest", bringing good tidings to both "him that gives and him that takes".
Portia says that mercy is divine, as it "droppeth...from heaven" and "an attribute to God himself". Mercy is a heavenly quality, a sacred virtue and he who has this characteristic becomes "likest God". It is like "gentle rain from heaven".
Mercy is fair treatment to others. For example, the Duke asks Shylock to "forgive a moiety of the principal", sympathising with Antonio as he has lost money in his wrecked investments.
Justice is strict and condemning, as the place where justice is practised is described as the "strict court of Venice". Portia asks Shylock to "mitigate...thy plea", exemplifying how justice is indeed harsh.