ESXi, vCenter Server, and Update Manager

ESXi

VMware documentation: http://www.vmware.com/files/pdf/techpaper/vSphere-5-ESXi-Operations-Guide.pdf
Core of the vSphere product
Bare-metal hypervisor (http://en.wikipedia.org/wiki/Hypervisor)
From v 5.0, only the "i" version with no service console is available http://www.vmware.com/products/vsphere/esxi-and-esx/compare.html
Ultralight footprint (70MB OS)

Make sure to read the ESXi maximums documentation (http://www.vmware.com/pdf/vsphere5/r50/vsphere-50-configuration-maximums.pdf) and memorize the key maximums:

1. # of virtual CPUs per host (2048)
2. # of cores per host (160)
3. # of logical CPUs per host with hyperthreading (160)
4. # of virtual CPUs per core (25)
5. Amount of RAM per host (2TB)

VMware vCenter Server

Centralized management utility for all ESXi hosts and their respective VMs.
Allows IT admins to deploy, manage, monitor, automate, and secure a virtual infrastructure in a centralized manner
VMware documentation: http://www.vmware.com/products/vcenter-server/overview.html
Available as a Windows application or as a prebuilt vCenter Server appliance based on Linux
Provides numerous capabilities:

• VM templates
• VM customization
• rapid provisioning and deployment
• role-based access controls
• fine-grained resource allocation controls
• vSphere vMotion
• vSphere distributed resource scheduler
• vSphere High Availability
• vSphere Fault Tolerance
• Enhanced vMotion Compatibility (EVC)
• Host Profiles
• Storage I/O control
• vSphere Distributed Switches
• Network I/O Control
• vSphere Storage DRS

vCenter Server is available in 3 packages:

1. vCenter Server Essentials (integrated into vSphere Essentials kits for small offices)
2. vCenter Server Standard ('normal' version)
3. vCenter Server Foundation (limited to 3 ESXi hosts and does not include vCenter Orchestrator)

vSphere Update Manager (VUM)

Helps keep ESXi hosts and VMs patched with the latest updates
Scans to identify systems that are not compliant
User-defined rules for identifying out-of-date systems
Automated installation of patches for ESXi hosts
Full integration with other vSphere features