views.py
from django.shortcuts import render_to_response
from django.contrib.auth import authenticate, login as auth_login

def login(request):
    msg = []
    if request.method == 'POST':
        username = request.POST['u']
        password = request.POST['p']
        user = authenticate(username=username, password=password)
        if user is not None:
            if user.is_active:
                auth_login(request, user)
                msg.append("login successful")
            else:
                msg.append("disabled account")
        else:
            msg.append("invalid login")
    return render_to_response('login.html', {'errors': msg})


html
    <form action="/login/" method="post">
        Login:&nbsp; <input type="text" name="u">
    <br/>
        Password:&nbsp; <input type="password" name="p">
        <input type="submit" value="Login">
    </form>
    {% if errors %}
        <ul>
            {% for error in errors %}
            <li>{{ error }}</li>
            {% endfor %}
        </ul>
    {% endif %}