a ERD is for a offline database, ie. not fields, entities...
a schema is the lil boxes with the lines going from the primary keys, it just visualy displays the relationships between the tables and the fields... im pretty sure an ERD is only for an offline dbase tho..
If I can remember correctly a schema is the overall plan of the database. An Entity Relationship Diagram (ERD) is a modelling tool used to show this overall plan by showing the relationship between tables.
