What is a volcano?
The word "volcano" comes from the Roman God of Fire, Vulcanus. Also the small volcanic island of Vulcano in the Eolian Islands off Sicily, was called after that god. Apparently, this island was highly active in Ancient times and people believed its crater was the chimney of the Vulcanus' forge,  where the hot lava and ash coming out from the crater were the visible evidence of his activity to forge weapons for the other gods.

What is the difference between an active, erupting, dormant and extinct volcano?
An active volcano is a volcano that has had at least one eruption during the past 10,000 years. An active volcano might be erupting or dormant.
An erupting volcano is an active volcano that is having an eruption...
A dormant volcano is an active volcano that is not erupting, but supposed to erupt again.
An extinct volcano has not had an eruption for at least 10,000 years and is not expected to erupt again in a comparable time scale of the future.

How do you know when a volcano cannot erupt anymore?
When there are no signs of an active magma chamber beneath the volcano (no unusual seismic activity, no volcanic gasses escaping etc.), and when there hasn't been any activity for a long time span (at least 10,000 years).

How would you find out if a volcano was going to explode?
The answer to this complex topic is the heart of the science of Volcanology and its ultimate challenge.
While there is no easy and short answer, this is the essence: you combine knowledge of the volcano's specific past behaviour with all available observation of its present state, and this allows you to make a long-term and a short term prediction:

  1. You study the volcano's eruptive behaviour in the past and ideally, you also try to find out wether there were any signs of change before its eruptions. Such changes might be: unusual seismic activity (i.e. earthquakes at the volcano), visible or otherwise detectable deformation of the ground (i.e. opening of cracks, swelling of the whole mountain etc.), changes in composition and temperature of fumarolic gases and so on.
  2. Then you monitor the volcano's behaviour in the present, looking for such changes.Based on the knowledge of the volcano's past, you can make a long-term prediction (example: sooner or later, Mount Rainier is going to erupt again, although nobody knows exactly when, but chances are almost certain high that this might happen within the next few centuries). The short-term prediction is possible when there are signs of change, and the more is known about the volcano and the more data are available about its present state, the more precise such predictions

How dangerous are volcanoes?
Volcanoes are usually less dangerous than other natural hazards such as earthquakes, tsunamis and hurricanes. 
But there is no good answer if you don't limit it into a specific context: which volcano? dangerous to what - people, property, etc.? during

Which type of activity? at which location?
Volcanoes have a serious of hazards (e.g. lava flows, ash fall, pyroclastic flows, climate changes on a global scale) that relate into different dangers or risks. The risks when visiting an active volcano depend on which risk zones of the volcano are visited and for how long.
Volcanic rock is an igneous rock of volcanic origin. Volcanic rocks are usually fine-grained or aphanitic to glassy in texture. They often contain clasts of other rocks and phenocrysts. Phenocrysts are crystals that are larger than the matrix and are identifiable with the unaided eye. They were created during fractional crystallization of magma before extrusion. Volcanic rocks are named according to their chemical composition. basalt is a very common volcanic rock with low silica content. Rhyolite is a volcanic rock with high silica content. Rhyolite has the same chemical composition as granite and basalt is compositionally equal to gabbro. Intermediate volcanic rocks include andesite, dacite, and latite. Volcanic rocks often have a vesicular texture, which is the result voids left by volatiles escaping from the molten lava. pumice is a rock, which is an example of explosive volcanic eruption. It is so vesicular that it floats in water. Pyroclastic rocks are the product of explosive volcanism. They are usually felsic (high in silica). Examples of pyroclastic rocks are tuff and ignimbrite. Shallow intrusions, which possess structure similar to volcanic rather than plutonic rocks are also considered to be volcanic.
Fumaroles are vents from which volcanic gas escapes into the atmosphere. Fumaroles may occur along tiny cracks or long fissures, in chaotic clusters or fields, and on the surfaces of lava flows and thick deposits of pyroclastic flows. They may persist for decades or centuries if they are above a persistent heat source (active Magma chamber) or disappear within weeks to months if they occur atop a fresh volcanic deposit that quickly cools. The teperatures rise from 70 C - 100 C or more. In some cases they are hidden in the ground (like in Stephanos-Crater on Nisyros) and you can break into them. The gases are dangerous and a gas-mask is often needed. They are alwas a sign of active volcanism.

How many volcanoes are there in the world?
The exact number of volcanoes is unknown. It also depends on the definition of a "volcano": for instance, there are "volcanic fields" that comprise hundreds of individual eruption centers  (such as conder cones, maars, shield volcanoes) that are all relataed to the same magma chamber and that may or not be counted as a single "volcano". 
There are probably millions of volcanoes that have been active during the whole lifespan of the earth. During the past 10,000 years, there are about 1500 volcanoes on land that are known to have have been active, while the even larger number of submarine volcanoes is unknown. At present, there are about 600 volcanoes that have had known eruptions during recorded history, while about 50-70 volcanoes are active (erupting) each year. At any given time, there is an average of about 20 volcanoes that are erupting.

Where is the krakatoa?
The krakatoa is located at Sunda Strait between Sumatra and Java island. it about 60 km away from West Coast of Banten

How to get there?
By boat, it takes about 1,5 hours if the weather is good

When is the best time to Krakatoa?
From April to October

Is there any Hotel and Restaurant at Krakatoa ?
No, there is no hotel or restaurant. we  stay an overnight at the tent and we bring and provide food for you