Express Edition (XE) is a free, downloadable version of the Oracle database server. Oracle XE can only be used on single processor machines. It can only manage up to 4 GB of data and 1 GB of memory. ISVs can embed XE in 3rd party products and redistribute it freely.
Oracle Standard Edition:
Standard Edition is designed for smaller businesses and enterprises. It offers a subset of the features/ functionality implemented in Enterprise Edition. Database options like Data Guard, Partitioning, Spatial, etc. is not available with Standard Edition (from 10g one can use RAC with Standard Edition). Standard Edition can only be licensed on servers with a maximum capacity of four processors.
Oracle Standard Edition One:
Standard Edition One is a low cost, entry-level version of the Oracle Standard Edition database server. Standard Edition One can only be licensed on small servers with a maximum capacity of two processors.
If you need more information you can refer to:
Francisco Munoz Alvarez