Helvetica is a very popular font. A standard with Adobe products, it’s a simple sans serif font that’s easy to read on screens and makes a wonderful body text. Microsoft did not want to spend the money to gets the rights to use it in their applications, so they designed a similar font: Arial. Since Windows 3, the font has been a standard for their office software product. The font became so popular that Adobe was forced to use it in it’s products, despite Arial being a cheap knock-off of Helvetica.
The fonts are very similar, especially in letter weight and spacing. In fact, if you typed a paragraph in Helvetica and then changed the font to Arial, you would not see any changes in word placement. To tell them apart, look at the lower case “t”. Its top is cut of at an angle in Arial, and straight in Helvetica. With “s” and “c”, the ends of the strokes are perfectly horizontal in Helvetica; in Arial they are cut off at a slight angle.