In order for a nation to have an effective nuclear deterrent they must be able to use that deterrent against an aggressor. If an aggressor is capable of wiping out all or most of a nations  nuclear arsenal in a surprise attack the nation would clearly be almost powerless to respond.

For this reason nuclear weapon-equipped powers do not "keep all their eggs in one basket". They have a variety of different ways to unleash nuclear weapons onto any would-be foe.
This is known as having a second strike capability.

The three delivery system types are :

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Strategic bombers such as the B-52, B1-B, Vulcan and Russian Tupelov Tu95 & Tu26 would have been used to penetrate enemy air space and deliver their deadly cargo onto strategic targets. With better radar and SAMs, the role of the long range bomber looked less important as its missions were made more difficult. 

Replica of "Little Boy" Atomic Bomb,  at IWM London

The late 1950's saw the development and deployment of ICBMs, thus reducing the role of the bomber still further. However, advances in stealth technology in the 1990's improved the bombers chances of invading enemy controlled air space. Both F-117 and B2 bombers have proven themselves in combat situations, and are clearly able to evade enemy radar installations to reach their intended targets.

Modern nuclear-bombers usually launch ALCM rather than actually dropping bombs. Each ALCM costs around $2,000,000. Non-nuclear, or conventional, ALCM were used against Iraq.

Intercontinental ballistic missiles (ICBM) are land based nuclear weapons. They were not introduced to the nuclear equation until around 1962. ICBM can travel up to twenty times the speed of sound, giving American and Russian missiles  a 30 minute flight time before reaching their targets. 

The early missiles only carried one warhead. Thus, one missile, one target. However more modern missiles carry numerous warheads, making them harder to destroy and more effective.

Mobile Missile Launcher on display at IWM Duxford

The USSR has more ICBM than America. They are bigger and can carry larger payloads. The USA missiles are smaller, but more accurate.

The START treaty has helped to reduce the total number of ICBMs.

Other land based nuclear weapons are known as tactical weapons. These have a lower yield, but can be delivered by short range missiles, bombs or long range cannon. A tactical warhead has approximately the same yield as the bombs dropped on Japan in WWII (20 kilotons)

Being able to stay submerged for up to two months allows nuclear submarines to prevent surprise attacks from any aggressor. A surprise attack could, theoretically destroy much if not all of the nuclear arsenal of a foe, however, as the location of the nuclear fleet is often unknown any aggressor would still be in for a nasty surprise should they attempt to attack.

Since the deployment of the Polaris submarines in 1963 they have consistently got faster and quieter.

Polaris Missile on display at the Imperial War Museum, London