A "giant" is defined simply as someone who is abnormally large, although the term is relative - a 6'2" person may seem like a giant to someone who is 5'2, but a modern-day 7'2" NBA basketball player would tower over them both.
Goliath is perhaps the most famous giant of Bible History, but there were numerous others, including some among the Israelites. Some examples of the often-related family branches of ancient giants:
Nephilim
Although their origin has been interpreted various ways, the Nephilim existed very early in human history. Independent Egyptian records of about 2000 BC also record their existence.
"And there we saw the Nephilim, the sons of Anak, who come from the Nephilim; and we seemed to ourselves like grasshoppers, and so we seemed to them." (Numbers 13:33 RSV)
Rephaim
The Rephaim occupied the land prior to the Canaanites, but were eventually defeated. The Anakim, Zuzim, and Emim people were related.
"For only Og the king of Bashan was left of the remnant of the Rephaim; behold, his bedstead was a bedstead of iron; is it not in Rabbah of the Ammonites? Nine cubits [i.e. about 13 feet / 4 meters] was its length, and four cubits its breadth, according to the common cubit." (Deuteronomy 3:11 RSV)
Anakim
The Anakim lived in the south, near Hebron. They were defeated by the Israelites under Joshua.
"And Joshua came at that time, and wiped out the Anakim from the hill country, from Hebron, from Debir, from Anab, and from all the hill country of Judah, and from all the hill country of Israel; Joshua utterly destroyed them with their cities." (Joshua 11:21 RSV)
Emim
The Emim were a warrior tribe of giants who were defeated by Chedorlaomer and his allies. Around the time of Abraham, they lived east of the Jordan.
"The Emim formerly lived there, a people great and many, and tall as the Anakim." (Deuteronomy 2:10 RSV)
Fact Finder: (a) Was Israel's first human king, Saul, an exceptionally tall man? (b) Was Saul's successor, David, through whom Jesus Christ is descended, of average height? What matters to God - the height, or the "heart"?
(a) 1 Samuel 9:2 (b) 1 Samuel 16:7,13