There are four kinds of tab stops and two types of margins, each indicated by a differently shaped icon in the ruler:
Right-pointing triangle: A tab stop that left-justifies text
Diamond: A tab stop that centers text
Left-pointing triangle: A tab stop that right-justifies text
Circle: A tab stop that justifies numbers around a decimal point
Horizontal bar: The left indentation for a paragraph’s first line
Downward-pointing triangle: The left or right indentation for a paragraph
If you don’t see the ruler, choose Format > Text > Show Ruler.