Disk encryption software
Disk encryption generally refers to wholesale encryption that operates on an entire volume mostly transparently to the user, the system, and applications. This is generally distinguished from file-level encryption that operates by user invocation on a single file or group of files, and which requires the user to decide which specific files should be encrypted. Disk encryption usually includes all aspects of the disk, including directories, so that an adversary cannot determine content, name or size of any file. It is well suited to portable devices such as laptop computers and thumb drives which are particularly susceptible to being lost or stolen. If used properly, someone finding a lost device cannot penetrate actual data, or even know what files might be present.
Encryption software
Often, the data in transit is between two entities that do not know each other - such as in the case of visiting a website. 
As establishing a relationship and securely sharing an encryption key to secure the information that will be exchanged, a set of roles, policies, and procedures to accomplish this has been developed; it is known as the public key infrastructure, or PKI. Once PKI has established a secure connection, a symmetric key can be shared between endpoints. 
A symmetric key is preferred to over the private and public keys as a symmetric cipher is much more efficient (uses less CPU cycles) than an asymmetric cipher.