Re: the installer of postgresql 9.4.1208 and 8.4-703 doesn't start

So I just downloaded and instelled SQL Workbench/J as my it department said, to help me with sql queries.

they also told me "Download a JDBC driver that will enable SQL Workbench/J to connect to your cluster. Currently we recommends this version 8 JDBC driver: http://jdbc.postgresql.org/download/postgresql-8.4-703.jdbc4.jar

I did it but I am not an expert, therefore I supposed I had to install it. Am I wrong?

a .jar file is a java package, they are used as-is, most typically by specifying them in the CLASSPATH when you start a Java program.   As Adrian said, in this case, you configure sql workbench/j to use it, rather than installing it in your OS like regular programs.

-- 
john r pierce, recycling bits in santa cruz