According to Wikipedia
WAP Push messages are SMS messages that alert the user, and allow the user to open the URL sent.

WAP Push, has been incorporated into the specification to allow WAP content to be pushed to the mobile handset with minimum user intervention. A WAP Push is basically a specially encoded message which includes a link to a WAP address. WAP Push is specified on top of WDP; as such, it can be delivered over any WDP-supported bearer, such as GPRS or SMS.

In most GSM networks, however, GPRS activation from the network is not generally supported, so WAP Push messages have to be delivered on top of the SMS bearer. On receiving a WAP Push, a WAP 1.2 or later enabled handset will automatically give the user the option to access the WAP content.

They are of crucial importance in mobile distribution. They allow users to supply their phone number and be sent a link to a WML page, XHTML page, a J2ME application, or on higher end handsets they allow the user to download a fully featured application.

There are two types of WAP push message: Service Indication (SI) and Service Load (SL). Service Indication messages contain a URI, a text string, a unique identifier and an action parameter: effectively, they present the URI and text string to the mobile device user and ask them if they would like to visit it. Service Load messages omit the text string; originally the intention was for the device to download the URI automatically, but for security reasons few devices do so. SI messages are therefore more common as they contain a text string.