There were ten "plagues" or major miracles performed through Moses or his brother Aaron in their confrontation with the Pharaoh of Egypt (not including Aaron's rod turning into a serpent - Exodus 7:10 - 12). An interesting thing to note is that the first three of these events (Nile River turning to blood, frogs and lice) affected both Egyptians and Israelites alike (the entire land). God protected the children of Israel from being affected, however, by his last seven judgments. Additionally, the magicians of Pharaoh's court could only duplicate, through their deceptive black arts, the first two plagues.
There are reasons why God choose the types of plagues he caused to come upon Egypt. One reason is as a testimony against the FALSE, counterfeit gods worshipped by the people (they worshipped the Nile, certain animals, the sun, etc.). Another reason is to show that he is the TRUE God who executes righteous judgment and is active in the affairs of humans.
The first four
The first miracle was turning the water in the Nile River AND the water in streams, rivers, ponds, and pools it fed into literal blood (Exodus 7:14-25). The Nile was worshipped among the Egyptians as a god because it provided water for growing food, drinking and bathing. God turned what they viewed as a giver of life into DEATH as a testimony against them. The waters of this great river were turned into blood that lasted for seven days (verse 25).
The second of ten plagues was frogs swarming out of the Nile River (Exodus 8:1-15). This event, like the first one, was mimicked by magicians (see Exodus 7:22, 8:7). Then came the trial of lice (gnats) upon both men and animals (8:16-18), a miracle that the enchanters of Egypt could not duplicate (verses 18 - 19). This was followed by the noisome onslaught of biting flies (verses 20 to 32).
The plagues of frogs, gnats and flies (as well as beetles, etc.) were all focused on what was worshipped by the Egyptians. In a twist of ironic (and somewhat humorous) fate, their own (false) gods were made to turn against and ATTACK them!
The next five
The fifth of ten plagues God brought upon Pharaoh (but spared his people) was a destructive pestilence among the cattle (Exodus 9:1-7). The contagious disease was so severe that it caused ALL the cattle to die (verse 6)! The sixth calamity was quite painful boils and sores (the KJV Bible calls them "blains") that came upon all Egyptians (verses 8 to 11). The seventh of the plagues against the people were thunder and hail that brought fire to the ground and caused all the plants to be smashed and every tree to be broken (verses 13 - 35).
The eighth judgment was a massive swarm of locust, which devoured any plant or tree that still existed in the nation (Exodus 10:1 - 20). This was followed by, since Pharaoh still refused to let God's people go (verse 20), by complete darkness that covered the land for seventy-two hours straight (verses 21 - 29). This was a testimony of God's power against the worship of the sun, which was prevalent throughout the nation.
The tenth and last
One of the many ways people try to discredit the Bible is by trying to "explain away" its miracles as naturally occurring phenomena. For example, some speculate that the destruction of Sodom and Gomorrah was not brought directly by the hand of God as judgment for their sins. They believe its demise came due to a random asteroid strike. Others argue that the parting of the Red Sea by Moses was not a miracle but occurred due to a strong and persistent wind. While the first nine plagues could be dismissed by unbelieving man as simply "acts of nature," the tenth and last one cannot so easily be explained away.
The firstborn of man and beast in Egypt were selected and killed, through a death angel, in a single night (Exodus 11:5, 12:12). The Bible states there was not a single house among the Egyptians that did not suffer the death of someone (verse 30)! Yet the Israelites who also lived in the land, and whom had lamb's blood over the entrance of their dwelling place, were spared the death of any human AND cattle (verse 13). The clear distinction between who lived or who died could only have happened through an act of almighty God.
It took ten plagues upon Egypt for Pharaoh to set free the children of Israel from bondage. Several Biblical commentaries, such as Adam Clarke's, estimate that more than three million men, women, and children were released from servitude through the mighty power of Jehovah.