1.类似于一个指针，但不是指针
2.在container中相当于指向element的指针，但其实现并不是指针，而是对象。 
3.从大的方面讲是容器和算法之间的黏合剂
从小的方面讲是类似智能指针的东东

试一下copy算法来加深理解
:)
4.iterator相当于一个container的指针类型，可以定义一个对象对container中的每个元素进行连续的访问。
5.你看一下源代码很快就明白了。我原来也晕了半天，实际就是指针，通过typedef实现。
template <class   T>   class   XXX
{
      ...
      ...
      typedef   T*   iterator
      typedef   const   T*   const_iterator;
};

建议：看C++ primer加深理解
源码参考：

实现:
1)Iterator.h
#ifndef ITERATOR_H
#define ITERATOR_H

typedef int DATA;

class Iterater;

// 容器的抽象基类
class Aggregate
{
public:
    virtual ~Aggregate(){}

    virtual Iterater* CreateIterater(Aggregate *pAggregate) = 0;
    virtual int GetSize() = 0;
    virtual DATA GetItem(int nIndex) = 0;
};

// 迭代器的抽象基类
class Iterater
{
public:
    virtual ~Iterater(){}

    virtual void First()        = 0;
    virtual void Next()            = 0;
    virtual bool IsDone()        = 0;
    virtual DATA CurrentItem()    = 0;

private:
};

// 一个具体的容器类,这里是用数组表示
class ConcreateAggregate
    : public Aggregate
{
public:
    ConcreateAggregate(int nSize);
    virtual ~ConcreateAggregate();

    virtual Iterater* CreateIterater(Aggregate *pAggregate);
    virtual int GetSize();
    virtual DATA GetItem(int nIndex);

private:
    int m_nSize;
    DATA *m_pData;
};

// 访问ConcreateAggregate容器类的迭代器类
class ConcreateIterater
    : public Iterater
{
public:
    ConcreateIterater(Aggregate* pAggregate);
    virtual ~ConcreateIterater(){}

    virtual void First();
    virtual void Next();
    virtual bool IsDone();
    virtual DATA CurrentItem();

private:
    Aggregate  *m_pConcreateAggregate;
    int            m_nIndex;
};

#endif
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2)Iterator.cpp
#include <iostream>
#include "Iterator.h"

ConcreateAggregate::ConcreateAggregate(int nSize)
    : m_nSize(nSize)
    , m_pData(NULL)
{
    m_pData = new DATA[m_nSize];
    
    for (int i = 0; i < nSize; ++i)
    {
        m_pData[i] = i;
    }
}

ConcreateAggregate::~ConcreateAggregate()
{
    delete [] m_pData;
    m_pData = NULL;
}

Iterater* ConcreateAggregate::CreateIterater(Aggregate *pAggregate)
{
    return new ConcreateIterater(this);
}

int ConcreateAggregate::GetSize()
{
    return m_nSize;
}

DATA ConcreateAggregate::GetItem(int nIndex)
{
    if (nIndex < m_nSize)
    {
        return m_pData[nIndex];
    }
    else
    {
        return -1;
    }
}

ConcreateIterater::ConcreateIterater(Aggregate* pAggregate)
    : m_pConcreateAggregate(pAggregate)
    , m_nIndex(0)
{

}

void ConcreateIterater::First()
{
    m_nIndex = 0;
}

void ConcreateIterater::Next()
{
    if (m_nIndex < m_pConcreateAggregate->GetSize())
    {
        ++m_nIndex;
    }
}

bool ConcreateIterater::IsDone()
{
    return m_nIndex == m_pConcreateAggregate->GetSize();
}

DATA ConcreateIterater::CurrentItem()
{
    return m_pConcreateAggregate->GetItem(m_nIndex);
}
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3_Main.cpp
#include "Iterator.h"
#include <iostream>

int main()
{
    Aggregate* pAggregate = new ConcreateAggregate(4);
    Iterater*  pIterater  = new ConcreateIterater(pAggregate);

    for (; false == pIterater->IsDone(); pIterater->Next())
    {
        std::cout << pIterater->CurrentItem() << std::endl;
    }

    return 0;
}
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