Moses or his brother Aaron performed ten plagues in their confrontation with the Pharaoh of Egypt. The first three of these events (Nile River turning to blood, frogs and lice) affected both Egyptians and Israelites alike. God protected the children of Israel, however, from the seven last plagues. The magicians of Pharaoh's court could only duplicate, through their deceptive black arts, the first two of these miracles.
Why did God choose certain types of plagues to fall 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 of the ten plagues was turning the water in the Nile River and the water in streams, rivers, and so on it fed into literal blood (Exodus 7:14 - 25). The Nile was worshipped in Egypt 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 became a plague of blood that lasted for seven days (verse 25).
The second of the ten plagues were frogs that swarmed out of the Nile River (Exodus 8:1 - 15). This event, like the first one, was mimicked by Pharaoh's magicians (see Exodus 7:22, 8:7).
The third plague was a trial of lice (gnats) upon both men and animals (8:16 - 18), a supernatural event that the enchanters of Egypt could not duplicate (verses 18 - 19). This was followed by the miracle of the noisome onslaught of biting flies (verses 20 to 32).
The frogs, gnats and flies (as well as beetles, etc.) of these miracles 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 fifth of ten plagues God brought upon Pharaoh and Egypt 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 painful boils and sores that came upon all Egypt (Exodus 9:8 - 11). The seventh of the plagues against the people was 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 plague was a massive swarm of locust which devoured any plant or tree that still existed in the nation (Exodus 10:1 - 20). It was followed by, since Pharaoh still refused to let God's people go to worship him (verse 20), by complete darkness that covered the land for seventy-two hours straight (verses 21 - 29). This was a testimony of the Lord's power against the worship of the sun, which was prevalent throughout the nation.
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 argue that the parting of the Red Sea was not a miracle but occurred due to a strong and persistent wind, or possible due to tides. While the first nine of Egypt's plagues could be dismissed by unbelieving humans as simply "acts of nature," the tenth and last one cannot so easily be explained away.
The tenth, and most profound, of the plagues that hit Egypt was the death of the firstborn of both man and beast. This was accomplished by a single death angel in one night (Exodus 11:5, 12:12). Every Egyptian home suffered the the death of someone (verse 30)!
The Israelites who lived in the land, however, and whom had lamb's blood over their door entrance, were spared such the tragedy of this "plague" (verse 13). The clear distinction between who lived or who died could only have occured through an act of 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.