If you are just after the tables and relationships in PeopleSoft, all table names are stored in PSRECDEFN.
To find all object references for each of these tables - ie fields, panels, panelgroups etc this can be done through application designer with 'find object references' , or you could just run the following sets of sql (not sure how much this changes between versions):
SELECT DISTINCT RECNAME,
FIELDNAME
FROM PSRECFIELD
WHERE FIELDNAME = 'xxx'
AND SUBRECORD = 'Y' ORDER BY RECNAME
SELECT DISTINCT PNLNAME,
FIELDNAME
FROM PSPNLFIELD
WHERE RECNAME = 'xxx'
ORDER BY PNLNAME
SELECT DISTINCT PNLGRPNAME
FROM PSPNLGRPDEFN
WHERE SEARCHRECNAME = 'xxx'
ORDER BY PNLGRPNAME
SELECT DISTINCT PNLGRPNAME
FROM PSPNLGRPDEFN
WHERE ADDSRCHRECNAME = 'xxx'
ORDER BY PNLGRPNAME
SELECT DISTINCT RECNAME
FROM PSRECDEFN
WHERE RELLANGRECNAME = 'xxx'
ORDER BY RECNAME
I believe there are sets of ERD's around depending on the versions if you are looking more at that level.