Hello, I took a Java language class, so I know how to write a code and different stuff..
now, I'm looking at job descriptions and some say they look for a server side developer and others for a client side.
does knowing Java language is enough to work at both? or was I taught only client side? I only know what I studied so I have nothing to compare it to, so I'm not sure what server side means in terms of what is actually required at this position.

They are probably referring to programs that:
- work without user interaction
- are extremely efficient. (subjective statement)
- work in a non-displayed (or minimally displayed (logs and debug output)) fashion
- communicate with data repositories and ensures the availability of resources
- possibly work in a machine-to-machine operation

Rather than code that:
- prompts a user for input
- validates a user inputs correct data types (ie. integer instead of s string)
- puts the data in user controls (drop-down lists, grids, labels, etc.) for user interaction
- is visually pleasing to the user
- is user friendly

There are those that do both.
You will find most programmers prefer one OR the other.

The line between server and client side programming is crossed (somewhat) when dealing with web content.

See:
Server-Side Scripting
Server-Side Programming