The SOLIDWORKS Online Help provides a summary of all of the SOLIDWORKS sketch relations available.
There is something missing from this table, it should show the icons associated with each relation. These are the symbols you see when you select the entities described under entities to select.
So I have copied the table from the link and added the missing images for your reference.
Relation  Entities to select  Resulting relations 

Horizontal  One or more lines or two or more points.  The lines become horizontal or vertical (as defined by the current sketch space). Points are aligned horizontally or vertically. 
Vertical  One or more lines or two or more points.  The lines become horizontal or vertical (as defined by the current sketch space). Points are aligned horizontally or vertically. 
Collinear  Two or more lines.  The items lie on the same infinite line. 
Coradial  Two or more arcs.  The items share the same centerpoint and radius. 
Perpendicular  Two lines.  The two items are perpendicular to each other. 
Parallel  Two or more lines.A line and a plane (or a planar face) in a 3D sketch.  The items are parallel to each other.The line is parallel to the selected plane. 
ParallelYZ  A line and a plane (or a planar face) in a 3D sketch.  The line is parallel to the YZ plane with respect to the selected plane. 
ParallelZX  A line and a plane (or a planar face) in a 3D sketch.  The line is parallel to the ZX plane with respect to the selected plane. 
AlongZ  A line and a plane (or a planar face) in a 3D sketch.  The line is normal to the face of the selected plane. 
Relations to the global axes are called AlongX, AlongY, and AlongZ. Relations that are local to a plane are called Horizontal, Vertical, and Normal.


Tangent  An arc, ellipse, or spline, and a line or arc.  The two items remain tangent. 
Concentric  Two or more arcs, or a point and an arc.  The arcs share the same centerpoint. 
Midpoint  Two lines or a point and a line.  The point remains at the midpoint of the line. 
Intersection  Two lines and one point.  The point remains at the intersection of the lines. 
Coincident  A point and a line, arc, or ellipse.  The point lies on the line, arc, or ellipse. 
Equal  Two or more lines or two or more arcs.  The line lengths or radii remain equal. 
Equal Curvature  Two splines.  The radius of curvature and the vector (direction) matches between the two splines. 
Symmetric  A centerline and two points, lines, arcs, or ellipses.  The items remain equidistant from the centerline, on a line perpendicular to the centerline. 
Fix  Any entity.  The entity’s size and location are fixed. However, the end points of a fixed line are free to move along the infinite line that underlies it. Also, the endpoints of an arc or elliptical segment are free to move along the underlying full circle or ellipse. 
Fix Slot  A slot sketch entity.  The entity’s size and location are fixed. 
Pierce  A sketch point and an axis, edge, line, or spline.  The sketch point is coincident to where the axis, edge, or curve pierces the sketch plane. The pierce relation is used in sweeps with guide curves. 
Merge Points  Two sketch points or endpoints.  The two points are merged into a single point. 
Doubled Distance  A centerline and any sketch entity.  The sketch entity is dimensioned at twice the distance from the centerline. 
Equal Slots  Two or more slot sketch entities.  The items have equal lengths and radii. 
On Edge  Edges of a solid.  The edges of the solid are projected to the sketch plane using the Convert Entities tool. 
On Plane  Sketch entities on a plane.  The sketch entities reside on the plane. 
On Surface  Sketch entities on a surface.  The sketch entities reside on the surface. 
Tangent to Face  A sketch entity and a solid face.  The sketch entity and face are made tangent to one another. 
Traction  See Using Traction and Belts for Layout Sketches. 
David Grossman says
Thank you for your post. I recently installed SW2015 and I’m noticing an annoying issue with the placement of the sketch relation items in the sketch. In previous versions of SW, the relation icons would be placed near the actual relation they were referring to. For instance, if two points have a symmetric relation, the icons would display near the points but not actually on the points. A horizontal or vertical relation would display next to a line bot not on it. Now, for some reason, the relation icons are displaying directly over the relations they are referring to. Sometimes they will obstruct each other if there are two relations at one point. I have not been able to find any way to change this in the options. Is it possible to change where the sketch relations display?