Migrating (moving residency from one country to another) has two sub-categories :
1. Immigrate : To enter a country of which one is not a native for permanent residence. 2. Emigrate : To leave one's place of residence or country to live elsewhere.
ex: For the last two years I have been thinking about migrating. I want to emigrate from the Philippines
US Census Bureau has been tracking migration patterns within the U.S. INS keeps track of immigrants who come in the United States. Statistics on emigration from the U.S. is kind of sketchy.
Another use of "migrate" is to describe the annual movement of certain birds and animals from one place to another -- "many birds migrate to warmer locations for the winter."