The Ark of Noah, most commonly known as Noah's Ark, was a large vessel constructed by Noah in order to protect his family and the animals of the Earth from a soon-coming Great Flood, which was foretold to him by God. God, seeing Noah as righteous man, forewarned him that he would destroy the Earth by means of a global flood, and gave him instructions for building the Ark to protect his family and designated animals.
Noah's ark landed in the mountains of Ararat. Along with two of each kind of "unclean" animal Noah brought seven others of each type of animal of "clean" animals for food and sacrificial purposes.
The NeedA Mixed Multitude
As the centuries progressed mankind became a mixture of good and bad tribes (and perhaps "nations"), which did not get along very well. However, sensuality lead to relationships that lead many away from God and towards the ways of Cain and his family. The leadership, especially, began to disregard God altogether. God determined that in could not last much longer.
During the time of Noah, humanity's population had greatly multiplied. Unfortunately humanity had become wicked and only did evil. God, regretting that he had created humans on the Earth, decided that He would destroy the Earth by global flood. However, God seeing that Noah was righteous and wanting to preserve not only humanity, but all the kinds of animals, told Noah to construct a large vessel, which is usually called an Ark. Due to the amount of destruction the flood would cause it would require a specially constructed vessel in order to withstand it, as the amount of water flooding the earth would quickly overwhelm any other floating vessel.
Though the Ark was indeed a ship, it is not referred to in the Genesis account as such, and rather is called an "Ark" (Heb: tebah, a waterproofed vessel). The only other "tebah" had one passenger -- the baby Moses. That "ark" was of reeds and waterproofed with slime. The Ark of the Covenant, which served as a vessel for God's Word, was gold-plated wood, and should not be confused with the tebah. That "ark" is an arown -- a container such as is used to hold valuables.
When Noah was shortly over the age of five-hundred he had his three sons Shem, Ham, and Japheth within that time period. When the flood was announced Noah's three sons had been born to him, and since Noah was six-hundred years old when the flood came this provides a period shortly under one-hundred years for the Ark's construction.
While God provided the architectural dimensions, and likely many of the engineering ingenuities of the structure, Noah himself had to construct the Ark, as commanded by God. It is likely that Noah received aid in constructing the massive vessel from his sons, and perhaps even his wife and his sons' wives. It may have been possible that other members of Noah's family such as his father Lamech and his grandfather Methuselah, but they both died before the Flood and so were not onboard the Ark. Even further Noah may have hired workers to assist in the construct that would have not been considered righteous.
Nevertheless the Ark of Noah was built within a little under a hundred year time frame and it may have taken this entire time frame, or only portion of it to build it. During this time Noah and his helpers also would have had to gather enough food to feed the animals and themselves that would have been aboard the Ark.
Boarding the Ark
After the Ark had been constructed, God told Noah and his whole family to go into the Ark, seven days in advance of the beginning of the flood. During this seven day period of waiting food would've been loaded on if not done so already. During this time all the designated animals came to Noah and boarded the Ark.
Going by directions from God, Noah began building the craft about 100 years before the flood. Its dimensions had a ratio of 300:50:30 in units called cubits, a measure of a man's arm from the tip of his fingers to his elbow. This has been assumed to be about 50 centimeters or 18 inches on the low side, and up to 22 inches on the high side. Built in three decks, the floor space would be 4500 square cubits (that is 3 x 50 x 300 cubits). Each deck had 15 cubits (about 16 meters or 22.5 feet) of head space.
The ship was built of a wood only mentioned once by name: "gopher" or "gofer". No one is sure what the wood was, though cypress (Greek: Kuparissos) grow in the area thought to be Noah's home base before the flood. To waterproof the wood, it was "covered with a covering (kopher)." This word is translated "pitch" in many English versions, but it seems the verb and noun are forms of the same word. What the coating was is not evident. The reader may note that the "g" and the "k" sound much alike.
There were eight adult humans -- Noah, his wife, Shem, Ham, Japheth and their wives. In addition two of most air-breathing animals and birds, and seven of those deemed "clean" (suitable to eat) animals. In storage was enough food -- quite probably dried fruit, along with seed and straw -- to last the journey.