Traditionally in SSL/TLS the client chose a master secret and encrypted it with the RSA key from the server's certificate. This is the approach used by the traditional RSA ciphersuites.

However the problem with this approach is that it if the server's long term private key is ever compromised then at attacker can passively decrypt the traffic. In particular they can go back to sessions that they recorded in the past and decrypt them retroactively.

So nowadays it is increasingly common to use "ECDHE-RSA" ciphersuites. In these the key exchange is performed by ephemeral elliptic curve diffe-hellman. The emphemeral bit means that the ECDH keys are only used for that one session. The RSA key from the certificate is used to sign the exchange to block man-in the middle attacks.

This means that someone who steals the server's long term private key is limited to active attacks on future sessions, they can't decrypt passively.

Traditionally in SSL/TLS the client choses a master secret and encrypts it with the RSA key from the server's certificate. This is the approach used by the traditional RSA ciphersuites.

However the problem with this approach is that it if the server's long term private key is ever compromised then at attacker can passively decrypt the traffic. In particular they can go back to sessions that they recorded in the past and decrypt them retroactively.

So nowadays it is increasingly common to use "ECDHE-RSA" ciphersuites. In these the key exchange is performed by ephemeral elliptic curve Diffie-Hellman. The ephemeral bit means that the ECDH keys are only used for that one session. The RSA key from the certificate is used to sign the exchange to block man-in the middle attacks.

This means that someone who steals the server's long term private key is limited to active attacks on future sessions, they can't decrypt passively.