Ground fire (slow, smouldering fire with no flame and little smoke),
• surface fire (where it is the litter and low-lying vegetation burning), or
• crown fire (where the fire moves rapidly through the canopy of the over storey or top layer of vegetation).
A bushfire is basically a fire that starts in the landscape outside towns, cities, and other such urban dwellings. A bushfire usually runs rampant, and can start naturally, or due to some catalyst, such as a human being leaving behind some inflammable substance, or due to someone leaving behind something on fire already, such as a fire that was not put out, or an unstubbed cigarette, etc. Sometimes a bushfire may be lit on purpose, e.g. to burn out existing vegetation for planting new vegetation. Sometimes it may be accidental, as mentioned above.
Basically, there are three types of bushfires. The most commonly occurring bushfire is the surface fire. These engulf the plants, grasses, scrubs, and other surface level vegetation in a landscape. The second type of bushfire is the crown fire. This is the most extreme sort of bushfire, as it engulfs the higher regions of a tree, and burn harder and faster because at that height the fire is fanned incessantly by the wind and also by the oil in the leaves of some trees. The third type of bushfire is the ground fire. Contrary to what the name suggests, this fire takes place just below the surface of the soil, working on the roots and other underground parts of the plants and vegetation. This fire is extremely dangerous because the fact it burns underground means it does not get noticed until it surfaces over the ground, by which time it might have caused extensive damage already.
Be more specific in your question, your wording could mean a couple of different things.
There are three types of "bush fires" defined as:
Ground Fires are fires that are in or under the ground. Tree roots and peat are good examples of this.
Surface Fires are fires on the ground, like grass and scrubs.
Crown Fires are the scary ones. This is when the tree tops or the tree crowns catch on fire.
Ground fires are very hard to put out as you generally can't see them. You may notice the ground change color or plant life die. They tend to pop out and turn into surface fires, sometimes after months of burning underground.
A crown fire can have two fire fronts, the first is the crown fire that races thru the tree tops, followed shortly after by a surface fire. Some people get caught out by this thinking the fire front has passed them.
- DJ, Firefighter from Victoria Australia. Bush fire capitol of the world!