Is it ok to lift weights every day?
It is not bad, but not good either. If you lift weights everyday, you should only do that for up to an hour. Your body cannot physically maintain a high level of energy to lift weights for longer than that. If you are thinking long term (as in lifting weights for a year or many months) then you should take every other day off. If you are trying to lift weights for a shorter time (3 months or less) and you want to lift weights as often as you can, take at least one day a week off (a good choice is Sunday).
However, if you work out too many days in a row, your body may get very sore. It is very wise to take at least one day a week off, if not 2-3. If you get sore, take a few days off in a row. You could end up tearing a muscle, ligament, or tendon if you decide to go through the pain. The saying "no pain no gain" is very incorrect. If you are experiencing pain, stop immediately. It is okay to feel fatigued, but pain is not good.
You shouldn't lift weights every day, though. You should mix the routine up. For instance, lift weights one day and do a cardio activity the next. Cardio activities include running, biking, going on the elliptical machine, doing a "stair master," or any other motion related activity. This will not only give your body a break from weight lifting, but will also allow your body to burn off calories in a way where you will build muscle and lose body fat.
You also shouldn't do the same weight lifting activities 2 days in a row. For instance, if you bench press, do arm curls, lower back extensions, and pull-ups one day, don't do any of those the next day. You need to let those muscles rest. The next day you should focus on the muscles that you didn't work out, like leg extensions, upper back exercises, curl-ups (or any abdominal exercise like the bicycle exercise), arm extensions (works the triceps), and dips.
Doing the max of any exercise is not recommended, though. The obvious danger in doing that is that you could easily pull a muscle, injure yourself, and possibly be too sore to work out for a few days. On whatever setting you do any exercise, you should be able to do 10 of them. If you are going to bench 25 lbs, you should be able to do that 10 times. If you are going to do a shoulder press on 50 lbs, you should be able to do that 10 times. A good strategy is to do 10, take a 30 second break, do 10 again (same machine), take a 30 second break, and do 10 more (still same machine). That way you will be able to fully maximize the machine and what muscle you are working and not just giving a small amount of effort.
If you are new to weight-lifting, don't overdo it on the first day. The first few days is the time where you will most likely injure yourself. It's definitely not fun when you think you can do a certain amount and come home only to find out that you are really sore and won't be able to work out for the next week. Make sure you stretch before you start lifting weights, and only do what you know you can do.
The biggest thing to understand about weight-lifting, or exercising in general, is that your body adapts very well. If you do 10 reps on 40 lbs. for anything, your body will be a little fatigued the first day (if 40 lbs. is a lot- it depends on what is heavy for you) and you will burn a good amount of calories. However, if you are still doing that after a week, you will have developed what is called "muscle memory" where your body will burn as little calories and use as little energy as it can to lift that amount of weights for that period of time. That is why it is very good to vary your workouts often. A pattern is good if you continually increase the weight level or reps, but if you know that you aren't going to, swtich the routine up at least every week. Here is an example routine in which somebody may do for a week. Note: All activities include 3 sets of 10 reps with 30 second breaks in between unless cardio.
Monday: Chest workouts (dips, chest press, bench press), arm workouts (bicep curl, arm extension).
Tuesday: Cardio (elliptical, running). Wednesday: Break (light cardio to stay in shape.)
Thursday: Abs (curl-up, total abdominal, bicycle kick), back (upper back workout, lower back extension), hips (dead lift).
Friday: Cardio (stationary bike, the wave).
Saturday: Legs (leg extension, leg press, leg curl, squats), shoulder (shoulder press, military press, vertical traction).
Sunday: Break (light cardio to stay in shape).
As the "Your body adapts well" section describes above, your body, well, adapts well. Make sure to vary when you workout what muscle, and how you work them out. Thus, you confuse your body and your workouts become very well done. You will notice results in at least 4 weeks if you truly follow these tips. However, it also requires you to eat well (i.e. no junk food every day). Eat lots of grains, fruits, vegetables, and lean meat. Doing so will really help your body stay in shape and get fit.