It is generally recommended to use an SSL certificate on your site to increase users confidence ( SSL is an encryption technology used to protect sensitive information passed between a web server and a web browser). If your website doesn't have an SSL, browsers will display warning messages, some of the newer browser versions pro-actively encourage visitors to close the page
There are many types of SSL certificates, depending on the validation type or the certificate type. The basic Domain Control Validated SSL certificates (which verify that the person who requests the certificate has “administrative” access to the domain ) are the cheapest ones and the easiest to validate and install on your domain. In the middle you have the Organisation Validates ones - the certificate authority manually checks some business identity documentation to validate the organisation behind the domain. At the top you have the Extended Validation ones which turn the address bar green - this is of particular significance in e-commerce scenarios where this directly translates to a measurable increase in sales as a direct result of the increased customer confidence.
There are also the single domain certificates - they cover a single domain, the multi-domain ones - which protect up to 210 domains hosted on a server and the wildcard ones - they protect all of the subdomains of your domain.
Long story short, SSL certificates increase users confidence and it's good to have them.