This may seem like a very basic question, but it is basic to determining the requirements of the application and, therefore, the best fit automation system. The way a product is manufactured, the performance needed, along with any physical limitations of the process, all influence the system selection.
Factory automation applications typically for which the PLC was originally designed, involve the manufacturing and/or assembly of specific items things. These PLC applications may employ one or more machines and a fair amount of material movement from machine to machine. A typical of this type of process characteristic is that the operator can usually monitor the things visually as they progress through the manufacturing line. The process is, by nature, very logic control intensive, often with high-speed requirements (throughput = profits). This type of process is often controlled by a PLC and Human Machine Interface (HMI) combination.
Typically, process automation applications involve the transformation of raw materials through the reaction of component chemicals or the introduction of physical changes to produce a new, different product, stuff. These applications may be built of one or more process unit operations piped together. The operator can't see the product is one key characteristic. It is usually held within a vessel and may be hazardous in nature. There is usually a large amount of simple to complex analog control (i.e., PID or loop control), although the response time is not that fast (100ms or greater). This type of process is often controlled by a DCS, although the analog control capability of a PLC may be more than adequate. A factor of determining in the selection process is often how large in scope the control application is (i.e., plantwide versus single unit and number of I/O points).
There may also be sequential (or batch) control needs. A PLC can be effectively used for simple batch applications, while a DCS is typically better suited for "complex" batch manufacturing facilities that require a high level of flexibility and recipe management.