Understaning postgresql instance vs database

In Informix, the instance is everything.   Buffer pools, transaction logging, backup and restores and all monitoring is done at the instance level
databases are just logical collections of tables in an instance.
In DB2 there is some instance configuration, but most everything is at the database level.   Buffer pools, logging, backups and restores are done at the database level.   Process load monitoring might be done at the instance level with db2top.
It seems like Postgres is like DB2, there are some things at the instance level and some at the database level.
But I'm having a hard time sorting out what is where.
Like what in PostgreSQL's statistics collector is an instance statistic verses a database statistic. 

I found a pgtop utility but it seems to require a database parameter
So are all stats database specific?or is the top info instance stats and the list portion database specific?