Spam, HTTPS, firewalls, cookies

Spam is unwanted bulk messages, mostly email, often advertising or scams. Email services filter spam into a separate folder, and users can mark messages as spam to train the filter.

HTTP and HTTPS. HTTP sends data between your browser and a website as it is, so anyone who intercepts it can read it. HTTPS (the S stands for secure) encrypts the data first, so intercepted data is unreadable. A website needs an SSL digital certificate to use HTTPS. So before typing a password or card number, check that the address begins with https://.

A firewall stands between a trusted network (such as a school's) and an untrusted one (such as the Internet), and filters the traffic passing between them according to a set of rules, blocking anything suspicious. It may be a network firewall (protecting a whole network, usually built into a router) or a host-based firewall (software protecting one computer).

Cookies are small text files that websites store on your computer. They have useful jobs:

• Session cookies remember what is in your shopping cart as you move between pages.
• Authentication cookies keep you logged in.
• Auto-fill cookies remember details you typed into forms.

But some cookies are threats. Third-party cookies, from sites other than the one you are visiting (usually advertisers), can track your browsing across many sites. Supercookies and zombie cookies are hard to delete, or recreate themselves after being deleted.