SSL (the predecessor of TLS) is a cryptographic protocol designed to provide communications security over a computer network. It is the S in HTTPS, IMAPS, SMTPS, etc.
The Secure Socket Layer is a set of protocols for cryptographically securing traffic. SSL was invented by Netscape to protect payment data in online e-commerce. Its successor tls is maintained by the IETF.
Technically SSL (more precisely: the Record Protocol) sits on top of the transport layer, but it is the responsibility of the application to use SSL. SSL can only be used with connection oriented transport protocol like TCP or SCTP.
Originally invented for HTTP traffic, SSL is commonly used for protocols like POP3, IMAP, SMTP, LDAP, and other protocols. SSL can be initiated by using individual protocol port numbers (such as port 443 for HTTP over SSL), the protocol names are recognized by the trailing letter S (such as HTTPS vs. HTTP). Other protocols allow to initiate SSL by the STARTTLS command.
Key-exchange and authentication is done by cryptographic public-key methods. Public keys are authenticated by chains of X.509 certificates that lead to root certificates which are ultimate trust anchors. For HTTPS common browsers come with a set of root certificates pre-installed.