There are four types of grenades available to four different classes:
Throws enemies away, in the same sort of manner as the Biotic Throw ability.
Breaks in to 3 parts (or 4 with one of the level six upgrades) each of which is relatively low damage (420-840) with moderate radius (4m), for an overall strong result. This is especially effective against a target caught between the separate pieces of the grenade, so careful aiming helps.
The seperate of the pieces make this grenade's real spread wider than 4m, so this grenade is good at crowd control. If positioned carefully, this can buy you time to manoeuvre or attack by displacing the enemy from their positions.
Sticks to a target or surface then explodes.
High damage (750-1500) and lowest radius (2-2.6m), with the potential to be armour piercing (+50% against armour) and used as a proximity mine.
The high damage output and stickiness makes these grenades pretty effective against a lone armoured target, but the radius makes them ineffective against groups.
Lifts enemies into the air.
Low damage (450-900 dmg) with moderate radius (4m), these grenades can effectively incapacitate your targets for a few moments (4-9 seconds).
Does weak damage compared to the other options, but the incapacitation of lifted enemies makes them very useful if you need time to get to cover, escape, manoeuvre, etc. As such this grenade should be used more tactically than the others - don't waste them trying to do damage unless you must.
Standard explosive grenade.
Good damage (650-935) with large radius (6.5-8.4m). At the top level these have +50% bonus against either shields or armour.
Best used against large, spread out, groups or when you're in a good position (as in, you don't need the time delay that a Cluster or Lift Grenade might buy you).
So, in brief, if you have a single big target to take down use a Sticky, but avoid wasting the other types if possible.
If you're facing a group and are out of position and you want to buy time (or the enemy is entrenched and you want to displace them) use a Cluster or Lift. Otherwise if you just need to hurt them, throw a Frag. Don't bother using a Sticky against a group unless you must.
Additionally, remember that grenades do not have a cool down period, so you can throw them in quick succession or while other powers are unavailable.
Best Answer
The Grenade indicator is not granular enough to give a good estimate of the potential "lethality" of any given grenade. Instead, it is a simple boolean indicating whether or not you're in range of some of the damage.
If you can see the grenade indicator, you will take damage when it goes off - but it won't tell you how much in advance.