According to some quick research via Google Search "Testimonials" replaced the old method of guestbooks on websites. It used to be signing a guestbook once you have visited a particular website.

This is not the practice anymore but is rather now replaced with "Testimonials".

The idea of testimonials is to receive positive comments on your service and to use this testimonial for marketing purposes.

Nowadays a "Testimonial" document is published on the website and a visitor is invited to submit a testimonial.

Besides the marketing effect of the testimonial content it also assists in two other aspects of your website's existence and value:

Customers always ask me how can they maintain their website constantly and one of my very first pieces of advice to them is to establish and make use of testimonials. It is a method to keep in touch with your customers and testimonials are not limited to one submission only, therefore constant content can be added to your website. 

When a customer submits his/her testimonial the chances are good that the customer used your products and services keywords to your website's SEO advantage.

How to put testimonials into practice?