The Goldilocks Zone
Earth orbits the Sun at a significant distance, which places it within a zone known as the Goldilocks Zone - also known as the Habitable, Comfort or Circumstellar Habitable Zone (which also includes Mars, but not Venus). This zone, as its’ various names suggest, is an area in which it is theoretically possible for a planet, with sufficient atmospheric pressure (due to it's mass), to maintain liquid water (and thus is limited to an average range within -15°C and 115°C). And as liquid water is essential for all known forms of life, planets within this zone are considered to be able to support life. If Earth was closer to the Sun, increased temperatures would trigger a ‘runaway greenhouse effect,’ and Earth would become like Venus today. If Earth was further from the Sun, reduced temperatures would mean that water would freeze (depending on the greenhouse effect), creating a ‘snowball Earth’.
Although the Sun is an incredibly reliable and constant source of light and heat, it has warmed-up, and continues to warm-up during its' lifetime. It is estimated that the Sun is now around 25 per cent hotter since life began on Earth , but it still remains 'just right' for life. And so, it has been hypothesised that Earth has actually shifted out of this 'goldilocks zone,' that was previously based purely on physics, and is now being collectively maintained through:
“...a self-regulating system made up from the totality of organisms, the surface rocks, the ocean and the atmosphere tightly coupled as an evolving system. The theory sees this system as having a goal - the regulation of surface conditions so as always to be as favourable as possible for contemporary life.” 
This is known as the Gaia Theory.
Gravity - The Curvature of Spacetime
“The biggest force that actually we are all subjected to, is the force of Gravity. It works 24hrs a day, the Sun, half the time. Gravity is the greatest force…” 
Gravity has built and created order in our solar system, galaxy and cosmos; forging dust and gas into stars and planets, and embracing them into interdependent orbits. Gravity on Earth is constantly reshaping, flowing water and nutrients downwards, eroding and pulling down mountains - dust, particle, rock by rock, and changing landscapes. But what is Gravity?
Gravity is a so-called ‘weak’ force, is one of four fundamental forces, and is the force of attraction that exists between all objects, which is proportional to their mass. This force between us and Earth, keeps us and our atmosphere, for instance, from floating out into space. Since it is proposed, through the equation E = mc2, by Einstein, that mass and energy are equivalent, then gravity also acts on energy (including light).
Gravity also works as a compressive force, acting on the mass of a planet or star for example. And like rain water wanting to flow downwards, all matter wants to flow downwards to the centre of the object - in this case a planet or star (always trying to move from a high potential energy state, to a low potential energy state). The ideal shape that allows this to happen is a sphere, and hence why planets and stars are round: not all objects in Space are round due to their mass - the greater the mass of the object, the greater the gravitational compressive forces ability to both compress and attract the outer surface to the centre to create a spherical form.
There is still much that is not fully understood about Gravity, but most current understanding is based around Einstein’s Theory of General Relativity. This is an explanation from Brian Cox:
“Gravity is the curvature of space-time [which is] the fabric of the Universe itself. What Einstein showed is that the presence of matter and energy - in the form of stars, planets and moons - curves the surface of space-time, distorting it into hills and valleys.” 
In the case of the Earth, our planet is falling into the valley of space-time created by the incredible mass of the Sun - the Gravitational Pull. And as Earth is trying to travel in a straight line perpendicular to the Sun - the Countering Velocity - maintains Earth caught in an orbit around the Sun. And what is true for the Sun and Earth, is true for all the planets in the solar system, the solar system within our galaxy, and all galaxies within the universe.