database question

Hi all I have been experiencing some strange behaviour on my postgres
DB.  I am VERY new to PG so bear with me as what I am going to ask is
all probably very basic to you guys.
First off over the last couple of weeks we have been seeing in the
dir /var/lib/pgsql/data/base/16450 some large file creations, so for
example

-rw------- 1 postgres postgres 1073741824 Sep 29 15:15 2683
-rw------- 1 postgres root     1073741824 Sep 29 15:15 2613.77
-rw------- 1 postgres root     1073741824 Sep 29 15:15 2613.83
-rw------- 1 postgres root       65347584 Sep 29 15:16 2613.88
-rw------- 1 postgres root     1073741824 Sep 29 15:16 2613.86
-rw------- 1 postgres root     1073741824 Sep 29 15:17 2613.82
-rw------- 1 postgres root     1073741824 Sep 29 15:17 2613.81
-rw------- 1 postgres postgres  380346368 Sep 29 15:17 16451.1
-rw------- 1 postgres postgres  217710592 Sep 29 15:18 33820
-rw------- 1 postgres root      119046144 Sep 29 15:18 2683.1
-rw------- 1 postgres root     1073741824 Sep 29 15:18 2613.84

What are these files and why have they suddenly started to be created
and why so large?