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Oracle Data Guard provides three protection modes including
Maximum Protection,
Maximum Availability, and
Maximum Performance modes.

(1) Maximum Protection Mode:

It provides the highest level of data protection which means zero (no) data lost will occur.
However, if connection to the standby database is lost, primary database might incur some downtime.
It requires online redo logs to be written and the SYNC redo transport  mode to be set on at least one standby database.

Oracle recommends that a minimum of two standby databases be used to protect a primary database that runs in this mode to prevent a single standby
database failure from causing the primary database to shut down.

Required Redo Transport Attributes:
a. AFFIRM
b. SYNC  
c. DB_UNIQUE_NAME

(2) Maximum Availability Mode:

It provides very high degree of data protection without compromising the availability of a primary database.
Transactions will not commit until all transaction data has been written to online redo logs and at least one standby database is synchronized within the maximum of NET_TIMEOUT seconds.

Required Redo Transport Attributes:
a. AFFIRM
b. SYNC   
c. DB_UNIQUE_NAME

(3) Maximum Performance Mode:  (default)

It provides the highest level of data protection that is possible without impacting a primary database performance.
Transactions are allowed to commit as soon as all transaction data has been written to the online redo logs.
Redo data are shipped to one or more standby databases asynchronously so that primary database performance will not be affected.

Required Redo Transport Attributes:
a. NOAFFIRM
b. ASYNC   
c. DB_UNIQUE_NAME

[Reference]

[1] http://docs.oracle.com/cd/B28359_01/server.111/b28294/protection.htm#CHDDAHJD
[2] Oracle Data Guard 11g Handbook