A square is present in a chart when two planets are 90 degrees from each other. The acceptable orb for a square is eight degrees. Another of the major Aspects, squares are generally considered to possess a hard energy.
Since squares are roughly three Signs apart (thanks to the 90 degree angle being formed), they are usually Cardinal, Fixed or Mutable in nature. Much like the qualities represented here, a Cardinal square denotes fast action, whereas Fixed squares speak to a more ponderous, measured state. Finally, Mutable squares can go either way, depending on other factors in the horoscope.
The essence of a square is friction. Things happen, and they're no small matter. The energy surrounding a square is forceful, intense and power-laden. It's effect on the individual will be to spur them to action, to cause decisions to be made. The risk to the individual influenced by a square is to ignore this Aspect's unmistakable energy. This Aspect will not be managed effectively until its force field and message are addressed. To illustrate the point, an individual with their Sun square Mercury might be expected to have difficulty when speaking. The lesson of this square will be to do the work necessary in order to speak more effectively.
A T-square is formed in the heavens when two or more planets are opposing each other and they square a third planet. In this formation, you have two squares present. Drawing a letter 'T' in the Zodiac is the easiest way to grasp this planetary formation. It is the third planet in this formation (the one not in an opposition) which is the focal point and the creator of the greatest tension (and by extension, the greatest opportunities). Lastly, a Grand Cross is formed when two pairs of planets are opposing each other. Think of a big plus sign dividing the Zodiac into four quadrants. In this formation, you see four squares. The tension created by a Grand Cross is quite strong and will surely challenge the individual. If it can be harnessed effectively, strength and success will be the result.