BelgianRN

Hey,
I'll give it a shot. CPAP is just continuous positive airway pressure. This means that the patients get a set amount of pressure (e.g. 5 cmH2O) applied to either his ETT or via mask. This 5 cm H2O gets applied regardless of inspiratory/expiratory efforts of the patient. There will always be 5 cm H2O applied. Conform the "rules" we speak of PEEP instead of CPAP when the patient has an ETT, and I will refer to PEEP after this. For this mode to work you need a conscious patient breathing in and out.
CPAP or just giving PEEP is mostly used in non-invasive ventilation where your patient has impaired oxygenation but can ventilate on it's own. A prime example would be someone in cardiogenic pulmonary edema. It will augment your patients oxygenation by keeping alveoli from collapsing and at the same time forcing some of the pulmonary edema back into the interstitium and keeping the edema localized. These patients have only a moderate alleviation of their work of breathing, but if you can get rid of the edema quickly enough your patient won't have to be intubated.
Pressure support is a mode where there is also a PEEP level set but the ventilator will deliver a preset pressure on top of this PEEP when it detects inspiratory efforts from the patient. This means for this mode you need to set at least two parameters (your PEEP and your pressure support level). And for it to work you need a breathing patient (so not heavily sedated).

Pressure control is used in the weaning phase of your patient where he does a lot of work himself to prepare him for the big bad world of breathing unassisted without a tube. It's also used during non-invasive ventilation when the problem is mostly respiratory failure due to muscle fatigue, loss of functional alveoli. Prime example is a COPD exacerbation that you want to keep from getting intubated (non-invasive) because part of their work of breathing comes from the added pressure support their muscles can rest a bit and recover for later when you DC the non invasive ventilation. Or the recovering patient that you are preparing to extubate (invasive).
Bipap stands for Biphasic positive airway pressure. When starting out I generally tell our newer colleagues to think of it as pressure control, but that's not completely right. I'll explain Bipap after explaining pressure control.
Pressure control is a mode where you apply a PEEP and an inspiratory pressure. The difference with pressure support is that you also add settings that determine when your patient will breathe in and out (e.g. frequency, I:E-ratio, etc.). Benefit of this mode is your patient has to do even less/no work breathing on their own and you can be sure that the machine will deliver its pressures when you want them to be delivered. Down side is your patient either needs to be sedated very well to allow the machine to determine when to breathe in/out or the patient's own breathing pattern will conflict with your predetermined settings and the patient will possibly fight against the machine.

An example would be your pneumonia patient that you intubated and you want to have him "rest" for a day (or two) as the antibiotics work their magic (you don't want to rest too long for fear of muscle wasting).
Most modern ventilators combine some form of pressure support and pressure control together so your patients gets X times per minute their pressure control mode but they can also trigger their own breathing that gets supported via pressure support in between. This leads to less fighting and discomfort for our patients, but can tire them out as well. I wish I could give you the terms but all different ventilator brands seem to have come up with even more creative names of naming all their modes.
I'm assuming you are using the Evita ventilator series. So Bipap in their system comes as either Bipap or Bipap/ASB.
Bipap is a "pressure control like ventilation" but instead of just delivering the pressure you set X times per minute it will allow a patient to breathe in/out on it's own (unassisted) on top of this inspiratory pressure level but also during the expiratory phase when there is just PEEP applied. So the idea is your patient fights less against the machine and will have less muscle wasting because he/she keeps using their breathing muscles.
Bipap/ASB is the same as Bipap but it will allow a pressure support (called ASB on an Evita) to be set as well. So when the ventilator detects inspiratory efforts from the patient it will give the ASB level of support instead of the inspiratory pressure you set for the pressure control part of the ventilator. During the expiratory phase when it gives its PEEP a spontaneous breathing attempt from the patient will also give them the ASB/pressure support level you set.
So Bipap has more diversity to it than simple pressure control. And you have different options. If you heavily sedate your patient that doesn't have their own breathing attempts your bipap becomes pressure control (you set PEEP, inspiratory pressure and frequency, and some other shizzle...). Since there are no attempts at breathing from the patient. It's pressure control.
If there is breathing but limited (and you didnt set an ASB/pressure support level) it will allow your patient to breathe in/out and regardless of this the machine will deliver its inspiratory pressure at X times per minute.
If the patient is breathing a lot (and you did set an ASB/pressure support) your patient will fall into a pressure support mode of ventilation doing more work themselves.
I hope this clears it up a bit. I can go on and on about vents but I'll keep it at that for now.