<% %>
Executes the ruby code within the brackets. It’s for any Ruby code, but doesn’t output the results (e.g. if statements).
<%= %>
Prints something into erb file. It’s for outputting the results of Ruby code.
<% -%>
Avoids line break after expression. It suppresses leading and trailing whitespace.
<%# %>
Comments out code within brackets; not sent to client (as opposed to HTML comments).
Visit Ruby Doc for more infos about ERB.
