Does Postgres automatically rotate WAL files out (pg_xlog) if archive_mode is off?

I'm relatively new to PostgreSQL administration and I'm attempting to pick up the nitty gritty details as I go. My question is, assuming I have no need for disaster recovery and I don't turn on archiving by setting up archive_mode = on, does Postgres have an internal mechanism to rotate WAL files out of pg_xlog? if so, what is the frequency? On a slightly different note, what is the impact of setting archive_mode = on but not configuring an archive_command - is there a default location Postgres will attempt to archive to?