What the differences of these two databases? Which one is better to use?

Basically both are Databases and have their own advantages.

MS-Aceess is a single application wherein you can create tables, queries, forms, reports, macros, modules, create custome menu bars, create relationships, create addins, web pages - everything will be in one place and you can just carry that one file - .mdb / .mde

The power of MS-Access lies in developing an application almost instantly.
The report that comes along with MS-Access is second to none.
A fairly experienced person in MS-Access can alomost instantly provide new forms, new reports and / or modify exisiting ones according to day-to-day requirements. Its comparably simple and easier to maintain the database.

Like all good applications, MS-Access has its own limitation too,
MS-Access if suitable for small to medium sized applications.
Their may be a limit to the size of the database too.
MS-Access is limited in terms of security and stability.
That is for MS-Access

Oracle is much biger is every sense, It is vast technology, It includes much more than just database management. Oracle is suitable for large applications, It is known for its stability and for its security. It takes an Expert to maintain oracle database.

Thanks for that information. May I ask another question. If oracle is better than MS Access, what database should I master in looking for a good job in database? Is MS Access have good pay also?

If considering platform independent DB, it would be good to get into Oracle else, You always have an option of SQL Server.

Choose between Oracle or SQL Server for a good careeer and good pay.
MS Access may not get you as many opportunities.
Even in terms of pay - Opting for MS Access may fetch you much lesser when compared to other major database.
All the while both Oracle and SQL Server are in a different league altogether.
We may not do justice comparing MS Access with the gaints.

In short to say each has its own advantage and the real usage of each depends on the needs. If one wants to have huge volumes in terms of usage, cost, security, users everything it is without any thought wise to make the selection of oracle. MS-access would prove good for small single application. So first make an idea about your usage, need and then go for it.