Character Large Object - data type used in Oracle and IBM DB2 RDBMSes to store strings of character longer than 4000 bytes.
XMLType is a system-defined data type for handling XML data in Oracle databases.
"A clob is a safer way to handle the soap request than an xmltype because the data returned may be longer than 32767 bytes"
from question "Parse a Soap Response in an Oracle 11g Function"
"Xmltype is being stricter about the validity than clob"
from question "Oracle mysterious Unicode codepoint"
"If you are storing varchar type data you should really be using one of the latter two types clob if you are storing various varchar data and xmltype which is a more specific type of clob anyway if you are storing strictly xml data"
from question "Select and Update Oracle BLOB column with XMLQUERY"