many-to-one 和 one-to-many怎么理解

<hibernate-mapping>
    <class name="com.cp.model.ProjComplete" table="CP_PROJ_COMPLETE" >
        <id name="cpId" type="java.lang.Long">
            <column name="CPID"  />
            <generator class="assigned"></generator>
        </id>
                <many-to-one name="proj" column ="PROJ_PK" />  
        <many-to-one name="modifyBy" column ="MODIFY_BY" />
        <many-to-one name="pm" column ="PM" />      
        
        <set name="comApplyAttach" >
            <key>
                <column name="CPID" />
            </key>
            <one-to-many class="com.cp.model.ApplyAttach" />
        </set>
        <set name="projDeptLink" >
            <key>
                <column name="PROJ_PK" />
            </key>
            <one-to-many class="com.proj.model.Proj" />
        </set>        
                <property name="modifyTime" type="java.util.Date">
            <column name="MODIFY_TIME" />
        </property>
        <property name="applyTime" type="java.util.Date">
            <column name="APPLY_TIME" />
        </property>
        <property name="expectedEndTimeStr" type="java.util.Date">
            <column name="EXPECTEDENDTIMESTR" />
        </property>
        <property name="isUrgent" type="java.lang.String">
            <column name="IS_URGENT" />
        </property>
        <property name="urgentDesc" type="java.lang.String">
            <column name="URGENT_DESC" />
        </property>
        <property name="completeReason" type="java.lang.String">
            <column name="COMPLETE_REASON" />
        </property>
        <property name="isValid" type="java.lang.String">
            <column name="IS_VALID" />
        </property>
        <property name="process_EndDate" type="java.lang.String">
            <column name="PROCESS_ENDDATE" />
        </property>
        <property name="cpStatue" type="java.lang.String">
            <column name="CPSTATUE" />
        </property>
    </class>
</hibernate-mapping>