Whatis

The whatis command takes a word or name as a parameter value that is searched against a set of database files that contain short descriptions of system based commands and devices.

mail# whatis fxp
fxp(4)                   - Intel EtherExpress PRO/100 Ethernet device driver

The search method used by whatis will find only complete word matches. This differs to the related apropos command which uses a search method that matches anything containing the given word. An example of these differences using the word "secure":

whatis

mail# whatis secure
_secure_path(3)          - determine if a file appears to be secure
rpc_secure(3)            - library routines for secure remote procedure calls
scp(1)                   - secure copy (remote file copy program)
sftp(1)                  - secure file transfer program
sprog(7)                 - secure programming practices
SHA1(3), SHA1_Init(3), SHA1_Update(3), SHA1_Final(3) - Secure Hash Algorithm

apropos

mail# apropos secure
_secure_path(3)          - determine if a file appears to be secure
krb5_verify_user(3), krb5_verify_user_lrealm(3), krb5_verify_user_opt(3), krb5_verify_opt_init krb5_verify_opt_set_
flags(3), krb5_verify_opt_set_service(3), krb5_verify_opt_set_secure(3), krb5_verify_opt_set_keytab(3) - Heimdal pa
ssword verifying functions
pam_securetty(8)         - SecureTTY PAM module
rpc_secure(3)            - library routines for secure remote procedure calls
scp(1)                   - secure copy (remote file copy program)
securelevel_gt(9), securelevel_ge(9) - test active securelevel
sftp(1)                  - secure file transfer program
sprog(7)                 - secure programming practices
SHA1(3), SHA1_Init(3), SHA1_Update(3), SHA1_Final(3) - Secure Hash Algorithm

This shows how whatis only found whole words for 'secure' whereas apropos found the word within other text, one of which was 'krb5_verify_opt_set_secure(3)'.

See also apropos command.