Squares amplify existing energies in order to create the maximum impact. They wake up sleeping beauties and volcanoes. It's the energy found in Apollo rocket engines which sent them into orbit. It is also the kind of energy building up in traffic jams and nuclear reactors. They can trigger chain reactions. It is the friction when you scratch a match. It's the fulmination when you are victim of a situation you don't have under your control.
On the positive side, squares can provide the energy required to finalize details or to galvanize situations. They can give a project or an artistic endeavor its final form, its final structure. It can also help starting them. Squares can fill a relationship with passion and activity, transforming boredom into fun and adventure.
In a birth chart or in synastry, they suggest some unchannelled and chaotic energy that will always be there. It needs your attention to be organized, evacuated or consumed using efficient, useful and non-destructive solutions. Square issues do not go away if you don't take care of them. Either they accumulate into tensions and blow everything away, or they slowly destroy everything on their path.
Square energies are nearly always manageable (even if it can require a lot of efforts and dedication). They rarely go completely out of control, unless they combine with other stressing aspects such as semi-squares, quincunxes or belong to Grand Crosses.
Related aspects: sesquisquares, semi-squares, t-squares and grand crosses
Other major aspects: conjunction, opposition, sextile and trine