The main difference is that double-quoted String literals support string interpolations and the full set of escape sequences.

For instance, they can include arbitrary Ruby expressions via interpolation:

# Single-quoted strings don't support interpolation
puts 'Now is #{Time.now}'
# Now is #{Time.now}

# Double-quoted strings support interpolation
puts "Now is #{Time.now}"
# Now is 2016-07-21 12:43:04 +0200

Double-quoted strings also support the entire set of escape sequences including "\\n", "\\t"…

puts 'Hello\\nWorld'
# Hello\\nWorld

puts "Hello\\nWorld"
# Hello
# World

… while single-quoted strings support no escape sequences, baring the minimal set necessary for single-quoted strings to be useful: Literal single quotes and backslashes, '\\'' and '\\\\' respectively.