Key usage:

Key usage extensionDescription
Digital signatureUse when the public key is used with a digital signature mechanism to support security services other than non-repudiation, certificate signing, or CRL signing. A digital signature is often used for entity authentication and data origin authentication with integrity.
Non-repudiationUse when the public key is used to verify digital signatures used to provide a non-repudiation service. Non-repudiation protects against the signing entity falsely denying some action (excluding certificate or CRL signing).
Key enciphermentUse when a certificate will be used with a protocol that encrypts keys. An example is S/MIME enveloping, where a fast (symmetric) key is encrypted with the public key from the certificate. SSL protocol also performs key encipherment.
Data enciphermentUse when the public key is used for encrypting user data, other than cryptographic keys.
Key agreementUse when the sender and receiver of the public key need to derive the key without using encryption. This key can then can be used to encrypt messages between the sender and receiver. Key agreement is typically used with Diffie-Hellman ciphers.
Certificate signingUse when the subject public key is used to verify a signature on certificates. This extension can be used only in CA certificates.
CRL signingUse when the subject public key is to verify a signature on revocation information, such as a CRL.
Encipher onlyUse only when key agreement is also enabled. This enables the public key to be used only for enciphering data while performing key agreement.
Decipher onlyUse only when key agreement is also enabled. This enables the public key to be used only for deciphering data while performing key agreement.

Extended Key usage:

Extended keyEnable for these key usage extensions
TLS Web server authenticationDigital signature, key encipherment or key agreement
TLS Web client authenticationDigital signature and/or key agreement
Sign (downloadable) executable codeDigital signature
Email protectionDigital signature, non-repudiation, and/or key encipherment or key agreement
IPSEC End System (host or router)Digital signature and/or key encipherment or key agreement
IPSEC TunnelDigital signature and/or key encipherment or key agreement
IPSEC UserDigital signature and/or key encipherment or key agreement
TimestampingDigital signature, non-repudiation.

Example of required key usage extensions

ApplicationRequired key usage extensions
SSL ClientDigital signature
SSL ServerKey encipherment
S/MIME SigningDigital signature
S/MIME EncryptionKey encipherment
Certificate SigningCertificate signing
Object SigningDigital signature

Source:
IBM Knowledgecenter