There are three arks in the Bible. Each one is a rectangular shaped container with a cover. It might help if we look at each one separately.
This one, of course, is easy. This is the ark of gopher wood that the Lord commanded Noah to build as a boat so that his family and the land animals might be preserved during the judgment of the flood (Genesis 6:14-19). Though it was large (300 cubits or at least 450 feet long), it matches the definition of an ark given above. It was a rectangular box-shaped boat with a cover.
MOSES' ARK OF BULRUSHES:
The ark of bulrushes was certainly tiny compared to the ark of Noah. It only had to carry one baby boy. However, it did float on water like Noah's ark (in this case, the Nile River). It was made of bulrushes and was used to preserve Moses from the command of the Pharaoh of Egypt to kill all the Hebrew baby boys (Exodus 2:1-4). We do not know its dimensions, but we can be confident that it had a rectangular box-shape seeing this is the shape that would hold a baby. We also know that it had a cover because when Pharaoh's daughter found this ark floating in the river, she "opened it" in order to see the baby (Exodus 2:5-6). However, this is not the ark that Solomon brought to the Temple.
ARK OF THE COVENANT:
The third ark was made of what was called shittim wood and was built at the commandment of the Lord as the most important piece of furniture in the tabernacle. Moses rode in the ark of bulrushes, but he had the ark of the covenant made at the commandment of God (Exodus 25:10-16). This ark was also shaped like a rectangular box. It's dimensions in cubits were 2 1/2 long by 1 1/2 wide by 1 1/2 high. That is approximately 3 feet and 9 inches long and 2 feet and 3 inches wide and high. It was actually quite small. It also had a covering on it. This significant covering is called the "mercy seat" (Exodus 26:34).
The ark of the covenant was hollow inside like a box and at times held various items which God commanded to be placed there. At one time it carried "the golden pot that had manna, and Aaron's rod that budded, and the tables of the covenant" (Hebrews 9:4). By the time Solomon moved it to the newly built Temple, it only had the two tables of stone on which Moses had written the Ten Commandments (1 Kings 8:9). It was a special type of the revelation of God's truth to man. Not only did it contain the Ten Commandments, but God promised Moses that "there I will meet with thee, and I will commune with thee from above the mercy seat" (Exodus 25:22). This ark that was built by Moses was the ark that was moved into Solomon's Temple.
Finally, I would like to mention the spiritual picture given by the three arks. Noah's ark is a picture of salvation. It is deliverance from the wrath of God. The ark of bulrushes is a picture of God's protection. It is deliverance from the wrath of man. The ark of the covenant is a picture of communion and fellowship. It is the promise that God will meet with us and commune with us if we seek Him. Each person needs each of these arks at some time in their life. I thank the Lord for each of them.