Congregation; Immigrants tend to live with people like themselves, which causes ethnic clustering. Ethnic groups may want certain services of their common background, such as grocery stores that offer their traditional food. Language communities may frequently form as well. Religion could also be a contributing factor. Congregated enclaves are subject to profile changes as they could follow where job openings are.
Segregation; Immigrants live together because discrimination forced or forces them to do so, causing ethnic enclaves to form. Segregation in the past could have caused a historical clustering of ethnicities that eventually formed "ghettos", which tend to stay put, either because they are forced or because of the already-present ethnic similarities where profile changes are not desired. Segregated areas could also be permanent because of available labor and refusal elsewhere.