Notes

Overall, here's the basics on White House Security:

OUTDOOR PERIMETER AND SECURITY MEASURES
The White House has a guarded outdoor perimeter. This means there is a very tall outer fence made of reinforced iron. The fence was initially built as a wooden fence in 1927 and since the 1990s, the perimeter has expanded more than an entire block. The purpose of this fence is to ensure traffic cannot get too close to the White House (to eliminate the possibility of car bombs near the physical building). Past the iron fence is a concrete blockade to make it harder for anyone slipping past the fence to get further into the grounds. There are also infrared sensors on top of a large amount of the fence posts. These sensors detect heat energy (ie. bodies). These sensor readings are watched by a security team in an undisclosed location. Supposing someone got past the iron fence and the concrete barriers without being detected by the sensors and guards watching the security cameras, they still have security personnel on the ground, snipers on the roof, and alarms underneath the ground with additional infrared sensors. The final precaution taken on the exterior of the White House is the windows--every single window is made of ballistic glass, which is bulletproof and each window has devices on them that prevent external listening devices from working (not sure how though, that information was not disclosed).

THE GUARDS
The Secret Service is a division of the Department of Homeland Security and they are the people who protect the President 24/7. They are often wearing undercover clothing, travel in packs, use code names, and have received special training to protect the President and the President's family for up to ten years after they've left office (the President has the option of taking Secret Service protection as well as how many agents they want as a part of their team).
Secret Service snipers (the ones positioned on the roof of the White House) are some of the best snipers in the world and must routinely hit targets accurately from 1,000 yards (about 1 km) every month to maintain their positions. The guards I mentioned on the ground--the ones that circle the White House to check for intruders--carry semiautomatic pistols, shotguns, and machine guns.

NO FLY ZONE
The White House and the surrounding area is a "No Fly Zone" -- this means that no unauthorized planes or helicopters can fly at any height above the White House and a set perimeter around it. The only planes/helicopters allowed within the No Fly Zone are specifically for the President and in the case of an emergency. To ensure this, the White House has laser detecting radar on the roof and the area is monitored 24/7.

VISITORS TO THE WHITE HOUSE
Every person and every thing entering into the White House must be fully screened before entering--this means any employee, food, packages, mail, and visitors.

GENERAL INFORMATION ABOUT THE SECURITY MEASURES
The White House security is built in layers--there are layers to the external security and layers to the internal security and each of those layers has other layers and those layers have layers which are randomized as often as possible.
What I mean by this is there is a series of physical barriers on the White House grounds and within these layers of barriers, there is a second layer of technological barriers and within this layer, there are guards and agents watching everything and creating perimeters and doing changing perimeter patrols (ie. there is a standard 4-block radius that is frequently patrolled but sometimes they'll go out and do 5 or 6 or 10 blocks to be safe and they'll also randomize HOW they do these patrols--sometimes it's drones above the city, sometimes it's plainclothes secret service officers, sometimes they'll look through security cameras and the cameras at traffic lights).

THE TUNNEL SYSTEM
In the early 1950s, under the Truman administration, the interior of the White House was rebuilt, reinforcing it with with concrete and steel beams. At the time of this reconstruction, a tunnel was built connecting the West Wing and the East wing and a bomb shelter.
In 1897, another tunnel was built under the Reagan administration. It allows the president to access a secret staircase outside the Oval Office by pressing on a wall panel to reveal and open a secret door. The passageway at the bottom of the stairs leads to a closet near the President's private elevator in the basement of the Residence.
There is a third tunnel, built as an air raid shelter by the Roosevelt administration, that leads from the basement of the East Wing to the basement of the Treasury Building.
The fourth known underground passageway provides pedestrian access between the White House and the Old Executive Office Building next door.
It is rumored that there are tunnels connecting the White House to the Capitol, Blair House, VP Residence, Camp David and the Pentagon, but none of these have been confirmed.