SAML is an XML based standard for exchanging authent/author information for SSO; that is, authenticate to one system and then get access to others without having to sign in again. SAML is typically used in browsers. So for SAML, think SSO.
SPML is a standard used for federated identity SSO. It used DSML, which allows it to present LDAP info in an XML format, which is useful for federated systems. So for SPML, think federations and LDAP integration.