The artificial nails types are determined by the materials are used to create it. There are a gel and acrylic technologies, both can be done with tips or forms.
Acrylic nails are durable, repairable in case of small cracks. For example, if you have a lot of typing on the keyboard, or you often have to work with water, then choose acrylic (but only if your nails are strong!), because this nail material survives in such "extreme" conditions, but the gel not.
Gel extension is more gentle, nails can breathe under gel. That's why if your nails are weak, fragile by nature - the best for you is the gel extension, though, comparing to acrylic, gel is less durable.
If the crack was got on the gel nails then the master will have to change the artificial nail, i.e. remove the old nail and build a new one. If a small crack appeared on the acrylic nails, it can be repaired, but if a crack is deep and goes right through artificial nail it have to be rebuilt as well.
But be very carefull and do not forget that extension can not be done if the nails are very weak and thin, because after that even strong and healthy nails become much thinner. In addition, if you have a fungus on the nails - it must be cured before making artificial nails.
То sum up, only you can make the right decision about the type of artificial nails and whether do it at all.
Make manicure for 3-4 days prior to the nail extension. This is very important to make top quality artificial nails and not feel pain during process - skin around the nail is very delicate and can easily damage by files. The break between the extension (after the removal old artificial nails) 5 - 8 weeks. You can wear new nails up to 2.5 - 3 months, then you have to remove them. Restoration, renovation nail should be done each 1.5 - 2 months. During this recovery period you should treat your nails carefully. Good for nail - bath with sea salt 2-3 times a week, bath with heated olive oil. If your nails are weak, fragile, it is better to avoid extension or make more gentle gel artificial nails in order to your nails can breathe.
After extension nails are very weak, thin - if you already have naturally weak nails, it is better not to make them worse and avoid that procedure. It is also a contra-indication to the making artificial nails are fungal nail disease (nail fungus and skin). You can not make nails extension right after the procedure of the manicure because the nails are nourished with moisture and the skin around the nails still more delicate and can easily injure by files.