apex:selectOptions
https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_compref_selectOptions.htm
A collection of possible values for an <apex:selectCheckBoxes>, <apex:selectRadio>, or <apex:selectList> component. An<apex:selectOptions> component must be a child of one of those components. It must also be bound to a collection of selectOption objects in a custom Visualforce controller

<!-- Page: -->
<apex:page controller="sampleCon">
    <apex:form>
        <apex:selectCheckboxes value="{!countries}" title="Choose a country">
            <apex:selectOptions value="{!items}"/>
        </apex:selectCheckboxes><br/>
        <apex:commandButton value="Test" action="{!test}" rerender="out" status="status"/>
    </apex:form>
    <apex:outputPanel id="out">
        <apex:actionstatus id="status" startText="testing...">
            <apex:facet name="stop">
                <apex:outputPanel>
                    <p>You have selected:</p>
                    <apex:dataList value="{!countries}" var="c">a:{!c}</apex:dataList>
                </apex:outputPanel>
            </apex:facet>
        </apex:actionstatus>
    </apex:outputPanel>
</apex:page>
  			
/*** Controller: ***/
public class sampleCon {
    String[] countries = new String[]{};
 
    public PageReference test() {
        return null;
    }
 
    public List<SelectOption> getItems() {
        List<SelectOption> options = new List<SelectOption>();
        options.add(new SelectOption('US','US'));
        options.add(new SelectOption('CANADA','Canada'));
        options.add(new SelectOption('MEXICO','Mexico'));

        return options;
    }
 
    public String[] getCountries() {
        return countries;
    }
 
    public void setCountries(String[] countries) {
        this.countries = countries;
    }
}

apex:selectList
https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_compref_selectList.htm
A list of options that allows users to select only one value or multiple values at a time, depending on the value of its multiselect attribute

<!-- Page: -->
<apex:page controller="sampleCon">
    <apex:form>
        <apex:selectList value="{!countries}" multiselect="true">
            <apex:selectOptions value="{!items}"/>
        </apex:selectList><p/>

        <apex:commandButton value="Test" action="{!test}" rerender="out" status="status"/>
    </apex:form>

    <apex:outputPanel id="out">
        <apex:actionstatus id="status" startText="testing...">
            <apex:facet name="stop">
                <apex:outputPanel>
                    <p>You have selected:</p>
                    <apex:dataList value="{!countries}" var="c">{!c}</apex:dataList>
                </apex:outputPanel>
            </apex:facet>
        </apex:actionstatus>
    </apex:outputPanel>
</apex:page>
            
/*** Controller: ***/
    public class sampleCon {
        String[] countries = new String[]{};
            
        public PageReference test() {
            return null;
        }
            
        public List<SelectOption> getItems() {
            List<SelectOption> options = new List<SelectOption>();
            options.add(new SelectOption('US','US'));
            options.add(new SelectOption('CANADA','Canada'));
            options.add(new SelectOption('MEXICO','Mexico'));
            return options;
        }
            
        public String[] getCountries() {
            return countries;
        }
            
        public void setCountries(String[] countries) {
            this.countries = countries;
        }
    }

Code for both

<!-- Page: -->
<apex:page controller="chooseColor">
    <apex:form>
        <apex:selectList id="chooseColor" value="{!string}" size="1">
            <apex:selectOption itemValue="red" itemLabel="Red"/>
            <apex:selectOption itemValue="white" itemLabel="White"/>
            <apex:selectOption itemValue="blue" itemLabel="Blue"/>
        </apex:selectList> 
    </apex:form>
</apex:page>
 			
 /*** Controller ***/
 			
public class chooseColor {
    String s = 'blue';
 
    public String getString() {
        return s;
    }
 			
    public void setString(String s) {
        this.s = s;
    }
}

1) https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_compref_selectOption.htm
2) http://sfdcsrini.blogspot.com/2014/07/how-to-use-apexselectoptions-and.html

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