Mold is a multi-cellular fungus, but it is considered microbic. Each mold organism is very small, the large mats of mold that you see are actually thousands of interconnected organisms. Each fungus will grow long filaments called hyphae which are used for just about everything. These hyphae spread out to form a network called a mycelium, or colony, which is considered to be one organism. Nutrients and even organelles can be transmitted through this network of hyphae, getting nutrients or anything else where they need to be in the organism.
There are thousands of species of mold that range from saprotrophs, molds that break down decaying matter for food, to thermophiles, molds that can live in extreme heat. Most molds secrete hydrolytic enzymes from the tips of their hyphae that break organic matter down into simpler compounds for the hyphae to absorb as food. Many molds also excrete mycotoxins which, along with the hydrolytic enzymes, discourage the growth of competition. Perhaps you've heard of penicillin? This life saving drug was derived from mold toxins, just going to show that mold is very good at keeping other organisms down.
What most of us are concerned about, however, is the effect that mold has on our health. Mold is pretty much everywhere, its spores float through the air and cling to your clothes, you can't escape it. Luckily mold only causes harm in large amounts. Some people are allergic to certain molds, these allergies can cause some pretty severe respiratory problems. Even if you aren't allergic to mold, the mycotoxins can still get you. Long term exposure to these bothersome toxins can cause neurological damage, and maybe even kill you. One of the biggest threats posed by mold is the infamous black mold that lurks behind the walls of flood damaged houses. Black mold isn't the only mold that can run rampant through your house and cause all sorts of problems, but it is the most widely feared.
Molds reproduce through spreading spores, as fungi are prone to do. These spores can be birthed through meiosis or mitosis, the latter actually being asexual. They are also very tough. Mold spores can survive extreme conditions for long periods of time, just waiting for the right conditions to grow. This is one of the reasons that mold always seems to show up. Some molds, xerophilic molds to be precise, can even remain hydrated off of humidity alone. Generally though, molds like a moist environment, the warmer the better. They have no problem with cold environments though, most molds can start growing at temperatures colder than your refrigerator, which is why your cheese always gets moldy if you don't eat it fast enough.
The outer layer of moldy cheese
can be cut away to make it safe for consumption.
Believe it or not, mold is even used to make food. For example, Koji molds were cultured in asia to ferment soybeans for soy sauce among other things. This same technique is also used to ferment rice to make various forms of alcohol.
Sake is a form of alcohol made from rice.
Despite all of its harmful effects, mold is the great recycler of the planet. Just about any organic substance can be broken down by mold, returning it back to the earth faster. Mold is just another vital part of Earth's ecosystem.