Justification is the manner in which you place a line of type. This first paragraph is left justified which is the way most text is justified. This is because we read text from left to right, so keeping the invisible line that the words line up with consistent on the left means it is easier for our eyes to find the beginning of the next line.
This next paragraph is right justified. Sometimes a design calls for a different treatment than the norm. Right justification can work great for headlines or short chunks of copy. More than that and the readability of the copy lowers.
This is centre justified. Centre justification is used mainly for headlines, but can be used for short passages of copy as well. Centre justification should be used sparingly as centring everything can look amateurish and just plain bad.
This last paragraph is full justified. This means that both the edges on the left and right have an invisible line for the words to line up with. To keep the spacing of words and letters from looking too awkward, sometimes a word will be split, using a hyphen (-), onto two separate lines. Something to be aware of and careful of with fully justified text is justified rivers. This is when negative space is created between words that flows into other lines creating a river look. This can be distracting and take away from the readability of the copy.
What is your favourite type of justification? I prefer left justification for copy and right justification for headlines!