All of the berries except for Starf raise a stat by one stage. Here is an explanation of stages, in this example, I will use a Pokémon that has an Attack stat of 200.
A Pokémon starts a battle with all of it's stats at Stage 0. The minimum stage for any stat until it can't be reduced any more is -6, and the maximum stage for a stat before it can't be boosted anymore is 6. Moves like Growl and the Intimidate ability will reduce Attack by 1 stage, Charm reduces Attack by 2 stages. Howl and Liechi Berry increases Attack by 1 stage, and Swords Dance increases Attack by 2 stages.
Ok, now here is a chart that explains how much the different stages effect the stat, once again, using a starting Attack stat of 200 as the example. Again, Pokémon start battles at Stage 0.
Stage -6: (2/8) 50
Stage -5: (2/7) 57
Stage -4: (2/6) 66
Stage -3: (2/5) 80
Stage -2: (2/4) 100
Stage -1: (2/3) 133
Stage 0: (1/1) 200
Stage 1: (1.5/1) 300
Stage 2: (2/1) 400
Stage 3: (2.5/1) 500
Stage 4: (3/1) 600
Stage 5: (3.5/1) 700
Stage 6: (4/1) 800
Evasion/Accuracy does not follow the exact same pattern though. All Pokémon start a Battle with 100% accuracy (unless they have the Hustle ability)
Accuracy/Evasion can be reduced/boosted in 6 stages, just like other stats. However, the reduction/boosting fraction is a little different.
Stage -6: (3/9) 33.3%
Stage -5: (3/8) 37.5%
Stage -4: (3/7) 43%
Stage -3: (3/6) 50%
Stage -2: (3/5) 60%
Stage -1: (3/4) 75%
Stage 0: (3/3) 100%
Stage 1: (4/3) 133.33%
Stage 2: (5/3) 166.67%
Stage 3: (6/3) 200%
Stage 4: (7/3) 233.33%
Stage 5: (8/3) 266.67%
Stage 6: (9/3) 300%
Now, if you are using a move that has less than 100% accurcy, then you multiply the accuracy reduction by the move's accuracy. For example: Say you are trying to use Toxic, which has 85% accuracy, and your accuracy has been reduced by 2 stages. (60%) Toxic would then have 51% accuracy, which is 60% of 85%.