This introduction is based on the Shulchan Aruch: Shin Shamech Beis (345).

A mechitza is a wall or fence.

Requirements

The minimum height of a mechitza is 10 tefachim from the ground.

The bottom of a mechitza must be connected to the ground.

A mechitza can not sway in a common wind.

Measurements

1 tefach is between 3.5 and 4 inches (there is a difference of opinions).

Principles

There is a principle discussed in the Shulcan Aruch called “Lavud.” Lavud means that any two objects within three tefachim of each other is considered connected.   In the case of a mechitza, if two walls are 3 tefachim away from eachother, they are considered connected.   Hence, we can allow for gaps (vertically and horizontally) in our walls of less than 3 tefachim.   Because of lavud, a picket fence is often acceptable to be used in a Eruv.

There is a principle called Omed Meruba Al HaParutz  which allows for gaps larger than 3 tefachim if the gaps meet certain criteria.  This principle is complex, and to understand where and how it can be used, one should speak to a Posek.