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I've refactored your code and added comments to explain certain things.

var expandAll, colapseAll;
$(document).ready(function() {
    // Since the $ function will query the document with the 
    // specified selector we want to cache these.
    // Doing this from within the ready callback allows us 
    // to keep these references hidden.
    var expandAllElement = $("#expandAll"),
        contElements = $(".cont"),
        triggerElements = $(".trigger");

    expandAll = function() {
        contElements.removeClass("hid");
        triggerElements.html("-");
        // Notice how we can chain these calls together?
        // jQuery was designed with a fluent interface.
        expandAllElement.html("Colapse All").one("click", colapseAll);
    };

    colapseAll = function() {
        contElements.addClass("hid");
        triggerElements.html("+");
        expandAllElement.html("Expand All").one("click", expandAll);
    };

    triggerElements.click(function(event) {
        // Always define a variable so the 'name' 
        // is not defined in the global object,
        var $this = $(this);
        $this.html($this.text() == '+' ? '-' : '+');
        $($this.nextAll(".cont").get(0)).toggleClass("hid");
    });

    // Using the one function allows you to skip 
    // unbinding the event handler for each click.
    expandAllElement.one("click", expandAll);
});