The two bulletins affecting IE are the highest priority. One of them, MS13-009, is referred to as the "core" IE update by Microsoft because it addresses a number of vulnerabilities in IE. It covers 13 bugs with all but one of them being Remote Code Execution vulnerabilities that can be used by an attacker to gain control over a user's machine via drive-by-download. That type of attack is common and is easily accomplished by surreptitiously installing malware on a Web surfer's computer when he or she visits a page with malicious code on it.
The second bulletin also for Internet Explorer, MS13-010, addresses a vulnerability in an ActiveX Dynamic-Link Library (DLL). It is rated critical and quite urgent to fix because the vulnerability is being exploited in the wild. The bug is in the VML (Vector Markup Language) DLL, the ActiveX control for the largely unused XML-based standard format for two-dimensional Vector graphics.