Difference between 'echo' and 'echo -e'

echo by itself displays a line of text. It will take any thing within the following "..." two quotation marks, literally, and just print out as it is. However with echo -e you're making echo to enable interpret backslash escapes. So with this in mind here are some examples

INPUT: echo "abc\n def \nghi" 
OUTPUT:abc\n def \nghi

INPUT: echo -e "abc\n def \nghi"
OUTPUT:abc
 def 
ghi

Note: \n is new line, ie a carriage return. If you want to know what other sequences are recognized by echo -e type in man echo to your terminal.

In most of the SHELL echo cant take escape sequence ( \n \t ). Where as echo -e can

echo -e " This is \n an \t example"

Single quote and double quote are mostly for handling the interpolation issues. You may find more details here, Why is echo ignoring my quote characters?