Difference One: Access is a small database system, which will allow you to create a small-medium sized database with minimum security features. Oracle can be integrated with NT security, which gives tight control over who can and cant get to your data.
Difference Two: Access is a lot slower over networks as all the processing is performed on the client machine, whereas with a large DBMS like Oracle (Or SQL Server), you send only a query (such as "SELECT * FROM Customers WHERE CustID=23;"), and only the required results are returned. If you ran this across a network with Access, all the records would be sent across the network, leaving the client machine to select the required record.
Difference Three: Access gets really slow if you try to connect more than 10 users (depending on the size of the DB and how it is being utilized), where are Oracle can accept hundreds/thousands of simultaneous connections (Depending upon the server it is sitting on).
The final major difference is that of triggers and stored procedures. Access can NOT store procedures which can be executed remotely. You can write functions and with a bit of "jiggery-pokery" get the server side to execute them vy setting flags or something in your database and getting the server to check at pre-defined intervals if the flag is set. With Oracle/(Other Large DBMS's) you can call the stored procedures and they will be executed on command on the server side. Triggers are like events on an Access form, such as "After-Update". When you update a record into Oracle, you can set triggers to automatically run procedures to check for pre-defined rules or whatever
