When linking out to other websites, be sure to select a quality website that you are linking to, note that you are giving a direction to your readers seeking more info, you do not want to upset your readers by linking to low quality web pages, sending reference links to well establish leaders in the industry for extended info related to the topic you are covering in the web page is a good practice, this can be handy for site visitors to explore in depth of the subject as well as helping a little on the search engine indexing process to categorise your web page. For example, your web page talks about orange, is there a well known leader that has detail information that you are not covering in your web page ? if so, in the web page, create a link within your web page paragraph and in the link description, try include a keyword / key phrase that relates to the web page theme as well as the target web page, which in this case 'orange' is the word that can be included in the link description.
Please note that, this is not a compulsory thing to do, but can ease hungry for info readers to explore further on the subject covering in your web page, note that you should not overdoing this in hope to bump up your web page in search engines, it can only do you harm more than good, only link out when necessary and naturally.
Back : No more than three level deep
Next : Internal cross link to related pages