Shapes are similar if they have the same shape (ie they have the same angles and their sides are of the same ratios) but are different sizes. This applies to shapes that have been reflected or rotated or moved.
So the two conditions for similar shapes are:

1. All angles are the same
2. All corresponding sides between the two shapes are of the same ratio.

For example the two shapes on the left are similar but the ones on the right are not.

These tri angles are similar because all sides are of the ratio 1:2

These triangles are dis-similar because sides are of differen lengths

The Sides and angles of one triangle can be worked out from another if they are known to be similar. The angles are all the same. To find the sides you must have one side which is known in both triangles. From this you can work out the ratio of the lengths of the sides of the two triangles and then calculate any unknown sides of one triangle if they correspond to known sides of the first by multiplying by the ratio.

The areas of similar triangles are also related. However because the area is two dimensional instead we must first square the ratio of the lengths of the sides before we can calculate the two areas.