Difference between base fact table and aggregate fact table ?

Hi I am new to data warehouse.
I want to ask two questions.
1) What is the difference between base fact table and aggregate fact table. I will appreciate if anyone explain it with example.
2) I have designed source schema (ER). Now I want to design dimensional model, what primary keys should be of each dimension ?

Thanks

Go here:
http://www.kimballgroup.com/data-warehouse-and-business-intelligence-resources/kimball-core-concepts/

Thanks but I could not get answer of my question-1 :-(

From Dictionary.com:

ag·gre·gate   [adj., n. ag-ri-git, -geyt; v. ag-ri-geyt]
adjective
1. formed by the conjunction or collection of particulars into a whole mass or sum; total; combined: the aggregate amount of indebtedness.
2. Botany .
a.(of a flower) formed of florets collected in a dense cluster but not cohering, as the daisy.
b.(of a fruit) composed of a cluster of carpels belonging to the same flower, as the raspberry.
3. Geology . (of a rock) consisting of a mixture of minerals separable by mechanical means.

An aggregate fact table is one made up of data from other fact tables.

ngalemmo wrote:From Dictionary.com:

ag·gre·gate   [adj., n. ag-ri-git, -geyt; v. ag-ri-geyt]
adjective
1. formed by the conjunction or collection of particulars into a whole mass or sum; total; combined: the aggregate amount of indebtedness.
2. Botany .
a.(of a flower) formed of florets collected in a dense cluster but not cohering, as the daisy.
b.(of a fruit) composed of a cluster of carpels belonging to the same flower, as the raspberry.
3. Geology . (of a rock) consisting of a mixture of minerals separable by mechanical means.

An aggregate fact table is one made up of data from other fact tables.

Appreciate this one.Thanks.