A ward is a magic defense set up by a magic user. There are a variety of different ones, ranging from killing to detection to stopping hostile magic. More powerful wards are difficult to make without a threshold. It is possible though to make a strong ward if there is a strong enough power source like a ley line.
The only known way for enemies to get passed this ward is to set the entire building on fire.
The most prominent example are the wards protecting the Harry Dresden's apartment, which appear to be organized in a double layer. The first layer is permanent and would do unknown damage to a tresspasser without an amulet or the skill to get through. The second layer is more like a panic room. It seals off his apartment making entry and exit impossible.
Another prominent ward is that around the White Council headquarters. Drawing on the unsual number of ley lines in the area it has been stated it would take a being on the order of a minor god to get through.
In the series
In "Ghoul Goblin", Joseph Talbot built a mansion that he intended to protect his family from the lethal family curse. The mansion was built in the form of a five point star, located at the center of a perfectly round iron fence, decorated with every magically defensive sign Joseph could find, including dreamcatchers and New Age symbols. Unfortunately, Joseph lacked any magical ability and thus couldn't create true wards.
However, his precautions did ultimately play a role in saving his family, as Harry Dresden was able to use the fence as a circle of power to create a defensive barrier against the creatures threatening the family.
In Ghost Story, Harry Dresden's ghost was bounced off a ward on Karrin Murphy's home nearly identical to his own. Karrin Murphy came out to investigate what triggered the ward. Dresden takes a closer look and saw a piecemeal sort of barrier instead of a solid wall. If some pieces were knocked out the rest of it would stand making it easier to repair. Dresden likened its appearance to LEGOS. Abby proudly claimed that she and the Ordo Lebes—Chicago's branch of the Paranet—built the wards around Murphy's house. It took 300 people from the Paranet working together in less than a day and two hundred pizzas.
Later on, the Corpsetaker as the Grey Ghost had built a complex arrangement of wards around her stronghold hidden within "quazimagical" script that looked nonsensical. So, when Dresden's ghost led a troop of Mortimer Lindquist's ghosts, including Sir Stuart Winchester, to rescue him they could not enter and were forced to go through her fortifications in the Nevernever.