Controller:

public class Sample {
    
    public List<ContactWrapper> allContacts { get; set; }
    public Boolean allChecked { get; set; }
    
    public Sample() {
        allContacts = new List<ContactWrapper>();
        allChecked = false;
        
        for(Contact con: [SELECT Name, Title, Department, Email From Contact LIMIT 10]){ 
            allContacts.add(new ContactWrapper(con));
        } 
    }
    
    public PageReference CheckAll(){
        
        for(ContactWrapper con : allContacts){
            con.selected = allChecked;
        }
        
        return null;
    }
    
    public PageReference ProcessSelectedContacts(){
        
        for (ContactWrapper contactWrapper : allContacts ){
            if(contactWrapper.selected == true){
                /* Do Some Processing */
            }
        }
        
        return null;
    }
    
    public class ContactWrapper {
        
        public Contact con{get; set;}
        public Boolean selected {get; set;}
        
        public ContactWrapper(Contact c){
            con = c;
            selected = false;
        }
    }
}

Visualforce Page:

<apex:page controller="Sample">
    <apex:form>
        <apex:dataTable value="{!allContacts}" var="c" id="contactsTable">
            <apex:column >
                <apex:facet name="header">
                    <apex:inputCheckbox value="{!allChecked}">
                        <apex:actionSupport event="onclick" action="{!CheckAll}" rerender="contactsTable"/>
                    </apex:inputCheckbox>
                </apex:facet>
                <apex:inputCheckbox value="{!c.selected}"/>
            </apex:column>
            <apex:column value="{!c.con.Name}" headervalue="Full Name"/>
            <apex:column value="{!c.con.Title}" headervalue="Title"/>
            <apex:column value="{!c.con.Department}" headervalue="Department"/>
            <apex:column value="{!c.con.Email}" headervalue="Email"/>
        </apex:dataTable>
        <apex:commandButton action="{!ProcessSelectedContacts}" value="Process Selected Contacts"/>
    </apex:form>
</apex:page>

Output: