a标签属性href中的mailto/sip/tel/sms作用

<a href='mailto:xxx@xxx.com'>Email</sip><br><br>
<a href='sip:xxx@xxx.com'>Skype</sip><br><br>
<a href='tel:131xxxxxxxx'>Telphone</sip><br><br>
<a href='sms:131xxxxxxxx'>Message</sip><br><br>