You want to know the software visitors use to access your site, for example, so you can optimize its appearance for the browser that most of your audience uses.
Add the following effector to your activity log format:
Request headers often include a field called the User-agent. This is defined as the name and version of the client software being used to make the request. For instance, a User-agent field value might look like this:
User-Agent: Mozilla/4.77 [en] (X11; U; Linux 2.4.4-4GB i686)
This tells you that the client is claiming to be Netscape Navigator 4.77, run on a Linux system and using X-windows as its GUI.
The User-agent field is neither required nor reliable; many users prefer software or anonymising tools that ensure that you can't tell what they're using. Some software even lies about itself so it can work around sites that cater specifically to one browser or another; users have this peculiar habit of thinking it's none of the webmaster's business which browser they prefer. It's a good idea to design your site to be as browser-agnostic as possible for this reason, among others. If you're going to make decisions based on the value of the field, you might as well believe it hasn't been faked because there's no way to tell if it has.