There are two key ones:
1. Microsoft's Patch Tuesday update fixes a bug in Internet Explorer 9 and 10 that is being exploited in the wild. If you have Windows Update enabled (and it's a very bad idea not to have this enabled) then this may already have been done. This isn't a problem if you have a Mac or Linux, or if you don't use Internet Explorer. If you use a company computer, and if your company has a big IT organization, then they may already have pushed this out to you.
2. Adobe has an emergency patch for Flash (the technology that powers video like Youtube). Be careful when you get their upgrade, in that they bundle a lot of bloatware (e.g. McAfee's Security Scan) with their Flash. You can click the box to turn this off, but you will want to pay attention during the install.
Both of these issues are rated critical, so you'll want to make sure you have the update.