A wildfire starts and spreads through various means, either natural or by human error, and grows to a point that isn't easily contained or extinguished. While these fires can happen anywhere there is fuel to burn, they are generally restricted to areas that are dry and of a certain type of tree or undergrowth that perpetuates their growth. Throw in a helpful wind or air circulation, and you have the perfect conditions for a wildfire.
The conditions needed for a wildfire's start is minimal, only requiring a calm day and something dry or flammable. Even after a light rain, if there are parts of a forest or field that are susceptible to prolonged heat from air temperature to foreign introduction, then it is possible the conditions to encourage the start of fire. In contrast, windy days may be even more likely to promote the development of fire, as even the smallest of embers or sparks can start a flame that will quickly turn into something uncontrollable.
The source of a wildfire can be many things, from campfires, to lit cigarettes, to electric sparks, to lightning, and to even dry friction between trees. When enough heat is applied to a flammable material, then it only takes the initial appearance of an open flame to spread. Whether the spread of the wildfire slowly radiates outward from the starting point, or travels a certain direction, it depends on the wind and the fuel available.
From sources like the campfire or cigarette embers, the start of fires will be low on the ground, snaking across dried leaves, grass, and fallen twigs, before climbing up undergrowth and small bushes. Eventually, the ground fire creates enough heat to light branches and leaves on fire, and the fires on the ground and through the upper portions of a tree help pass the fire to other full trees as the blaze pushes onward, in search of new fuel sources to burn. Meanwhile, the trees that have already been passed are left to bathe in the residual flames, will provide a constant source of heat for the front line of the fire, in order to dry it better. Embers, sparks, and hot ashes will be bourn upon winds and deposited elsewhere, either ahead of the fire or beside it, and start the growth in a different location. In the case of natural starts, fire from lightning may originate on the ground or in the branches of a tree, but usually falls to the ground and grows there in the same way.
Fires continue to spread with the wind and hardly ever against it, until they encounter a blockade in the form of a large river or lake. Sometimes wind bourn starters can ignite a blaze on the other side to keep it going, and sometimes rain or calm conditions will help extinguish it naturally, or the fire burns everything there is to burn and stops when there is nothing left to feed it.