There are many different types of germs; the main ones are bacteria, viruses and fungi.
Bacteria are very small living organisms. Bacteria are all around us all the time! However most bacteria cannot cause disease - some bacteria are even good for us, helping us digest food. Bacteria have always been with us, some cause specific diseases such as tuberculosis and cholera. Other bacteria cause a more general infection with symptoms such as diarrhoea and/or vomiting e.g. Campylobacter, Salmonella and E Coli.
Viruses are incredibly small - 100 times smaller even than bacteria. 100,000 of them laid end-to-end would make a line 1cm long! Alone a virus is not a complete living thing. It needs to infect a host cell of a living thing to reproduce. Unlike bacteria, which require food and water or they die, viruses can remain dormant on dry surfaces for a relatively long time before being picked up by a new host - some flu viruses have been shown to survive on hard surfaces like plastic for between 24 and 48 hours.
Like bacteria, Viruses can cause either specific diseases such as chicken pox, hepatitis or measles; which are easily recognised by their symptoms. Or they can cause more general illnesses which have non-specific symptoms such as: respiratory infections e.g colds (rhinoviruses), flu (influenza virus) and gastroenteric infections eg rotavirus (of which the main symptom is diarrhoea) and norovirus.
As you can imagine with these general symptoms it's often difficult to tell whether the cause of an illness is a virus or bacteria.
Although they're small, viruses can be incredibly powerful. Norovirus needs as little as 10-100 tiny particles to cause infection but a vomiting attack caused by this virus releases an estimated 30 million virus particles into the air.
Fungi are the most visible germs and very common. One survey found that 46% of households studied had visible mould.
There are 2 types of Fungi: moulds and yeasts.
Mildew, which is a common mould, often causes a black or green discolouration of walls and grouting. Typically in those hard to reach corners. Whereas if your shower curtain is covered in pink gunge then you most likely have a type of yeast living there.
Moulds love damp environments such as the bathroom. As well as looking horrible they can give off nasty smells and the spores they emit (the way they reproduce and spread) can cause allergic reactions eg hayfever or bronchitis and sometimes even asthma attacks.