Defender is a free-to-download piece of software that was developed by Microsoft for use by any of its customers who are running a licensed version of their Windows operating system. It was based on a system first developed by the GIANT Software Company.
Defender is not a firewall. Microsoft has a separate programme which performs this function called Windows Firewall. Since the launch of Windows 10, however, both systems have been bundled into a single package making the distinction redundant for all but the most advanced of users.
It is fair to say that Defender prevents large amounts of malware from finding their way onto the operating system of many personal computers and other devices. However, like all anti-malware systems, it cannot be guaranteed to ensure your system does not become infected with malicious software.
Yes, along with other security measures, Windows Defender is an antivirus programme. What's more, it is a good one which means that the need for a third-party antivirus programme to be installed on your computer is greatly diminished. It should protect your PC from other nasty things, like spyware, too.
It is highly likely that if you choose to run an established antivirus programme, such as McAfee, on a PC that already has Defender running on it that there will be problems. Each programme may try to uninstall the other. It is better to opt for one or the other and not both since your system's resources will soon get used up otherwise.
Google Chrome is a web browser. Defender is a security system. They do different jobs and don't get in each other's way, therefore. If you have Chrome installed on your computer then you should find no issues using it at the same time as running Defender.
Emails that arrive on your computer via an email server, such as Hotmail, are not scanned or read in some way by Defender. However, any attachment – the thing that can cause harm to your PC – is scanned by the system if you click on it. This helps to prevent email-borne viruses from spreading.
Defender does not perform all of the same tasks for Windows 7 users that Windows 10 users enjoy. However, its anti-spyware function does work and is useful. For better protection, Microsoft advises that Windows 7 users download the Microsoft Security Essentials Package to run alongside Windows Defender. This is available without charge.
If you turn off the system, then you are liable to download a virus without real-time protection. If you are doing something safe, then this should cause no problem for you and you might want to temporarily switch it off to perform certain tasks.
If you have turned the programme off temporarily, then it should come on once more when you restart Windows.