PK is defined in context of a table$($entity$)$ so that it must uniquely identify a row in the table. I.e$.,$ if i give you the value for PK it should return one and only one row from that table. PK can be a single attribute or a combination of attributes.

FK: is defined as an attribute in one relation which must be a Pk$($or part of PK$)$ in another relation. It is used to join $($reference$)$ tables.

Given the following relations identify PK and FK of each relation

CUSTOMER $($cust$\_$ID$,$ cust$\_$name, cust$\_$address, cust$\_$balance$)$

Now let's assume a new attribute account$\_$ID is added

CUSTOMER$\_$ACCOUNT$($cust$\_$id$,$ acct$\_$id$,$ cust$\_$balance$)$