Let's say you have a web server that produces HTTPS web pages, such as https://www.example.com. When a client submits a request to get a web page from your web server, a public certificate and it's corresponding private key in a keystore on the web server can be used for the SSL handshake and to then encrypt the packets being exchanged between the client and the web server. A keystore is a file such as foo.p12 or bar.jceks.
Let's say you know you are going to be requesting a resource from a server often over a secured channel, such as HTTPS. Instead of going through the SSL handshake negotiation process to have the server provide you with the public certificate from the servers keystore, you could store the servers public certificate in a truststore. Then, when you need to request a resource from the server, you will already have the certificate, so that you can present the certificate to the server and get on with the request. A truststore is a file, such as trust.p12.
The Java keytool command can be used to view, import, add, and remove public certificates and private keys from a keystore or truststore. In this example, the key.p12 keystore contains a public certificate called "default". A private key doesn't contain user specific data, such as an "alias" or "expiration date", so the user specific data in the keystore represents the public certificates in the keystore, not the private key.
~]# keytool -list -v -storetype PKCS12 -storepass changeit -keystore /path/to/key.p12 . . . Alias: default
In this example, the trust.p12 keystore contains a public certificate called "example.com".
~]# keytool -list -v -storetype PKCS12 -storepass changeit -keystore /path/to/trust.p12 . . . Alias: example.com