Copy and paste the following script.
<script language="ecmascript" type="text/ecmascript"> 
        var groupCollection;
        function getSiteGroups() {
            var clientContext = new SP.ClientContext();
            this.groupCollection = clientContext.get_web().get_siteGroups();
            clientContext.load(groupCollection);
            clientContext.executeQueryAsync(Function.createDelegate(this, this.onQuerySucceeded), Function.createDelegate(this, this.onQueryFailed));
        }
        function onQuerySucceeded() {
            var groupName = 'Site Groups: \n';
            var groupsEnumerator = this.groupCollection.getEnumerator();
            while (groupsEnumerator.moveNext()) {
                var group = groupsEnumerator.get_current();
                groupName += group.get_title() + '\n';
            }
            alert(groupName);
        }
        function onQueryFailed(sender, args) {
            alert('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
        }</script>
    <input id="btnGetSiteGroups" onclick="getSiteGroups()" type="button" value="Get Site Groups" />