This is quite a large topic to cover but edited highlights are
If you want to repeat content on every page, like a header or navbar for example then put it in layouts/application.html.erb
if you want to keep your views tidy with content specific to that page then you can use partials. Normally in views I create a folder called shared and then put my partials in there. for example if i save a partial called _form.html.erb (make sure you name partials with the underscore at the beginning of the file name), i would then call that in my view like
<%= render 'shared/form' %>
you can also use partials in the layouts/application file to keep that clean aswell
This is just my way of doing it, I'm sure more experienced rails guys have better ways
Hope that helps, any more questions then just ask