What ports are in the rear of my iMac 21” Mid 2010?

For connection information, see the following from: iMac (21.5-inch, Mid 2010) - Technical Specifications

Connections and expansion

• One FireWire 800 port; 7 watts
• Four USB 2.0 ports
• SDXC card slot

Also

• Mini DisplayPort output with support for DVI, VGA, and dual-link DVI (adapters sold separately)
• Headphone/optical digital audio output (minijack)
• Audio line in/optical digital audio input (minijack)

For the Mini DisplayPort output, the maximum supported resolution is 2560x1600 pixels (which was the resolution of some old 30" monitors).

including a Gigabit Ethernet port.