<% %>
Executes the ruby code within the brackets.
<%= %>
Prints something into erb file.
<%== %>
Equivalent to <%= raw %>. Prints something verbatim (i.e. w/o escaping) into erb file. (Taken from Ruby on Rails Guides.)
<% -%>
Avoids line break after expression.
<%# %>
Comments out code within brackets; not sent to client (as opposed to HTML comments).
Visit Ruby Doc for more infos about ERB.