That is a loaded question and one that will not be easily to answer here but I will try to give you an overview and then point you to some documentation that will fill in the holes - The networking is handled by the use of virtual switches. There are two trypes of virtual switches - Standard Virtual Switch and Distributed Virtual Switch. The standard virtual switch is simpler and one is created when ESXi is installed - The distributes switch is a feature of vCenter but provides more control over the networks traffic including a more granular traffic shaping tthan what is provided by the standard switch. The physical NICs are assigned to the virtual switches and through the switch configuration you will be abe distriubute the traffic across the physical nics -
To these switches your will attach either virtual machine port groups or VMkerne; ports. Virtual Machine Port Groups is how you will connect the VMs so they can communicate to the physical network. The VMkernel ports can be configured for management traffic, vMotion traffic, Fault Tolerance traffic, or storage traffic.
Here is a link for more information - http://pubs.vmware.com/vsphere-50/index.jsp?topic=%2Fcom.vmware.vsphere.networking.doc_50%2FGUID-35B40B0B-0C13-43B2-BC85-18C9C91BE2D4.html