It is very important to keep your version of wordpress and the plugins up to date.
New versions are released on a regular basis to not only update features but also to close any security holes.
A plugin that can detect if your site has been infected and help tighten up security.
Reminders will be sent when plugins fall out of date or the wordpress core falls out of date.
These will prevent a person or system from getting into your website by limiting the number of login attempts that are allowed.
Once the limit is reached the login will be inaccessible for a period of time.
One of the two plugins that comes by default with Wordpress, Akismet helps to limit the amount of SPAM commenting that happens through the system.
You must sign up as part of the community in order to use this plugin. When an item is marked as SPAM it lets all of the members of the wordpress community know and prevents future SPAM from that IP address.