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A run-on sentence (aka. fused sentence) is a sentence which joins two or more independent clauses without a conjunction or an appropriate punctuation.  Comma splice, which joins two independent clauses with a comma, is a type of run-on sentence. 

A run-on sentence is a grammatically incorrect sentence.  A sentence can be a run-on sentence if it falls under the above criteria, no matter whether it is a short sentence or a long one.   

Examples of run-on sentences:

1.  Mary laughed, she went inside.  (X: comma-splice error)

2.  Mary laughed she went inside. (X)

3.  Mary won the race, however, she paid a very high price for that.  (X)

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How to avoid run-on sentences?

a.      Find out if your sentences have two separate subjects and two separate verbs (excluding auxiliary)?

b.      Find out if your sentences have two independent clauses joined by a comma.

How to fix run-on sentences?

a.      Place a period between the two independent clauses.  (Start a new sentence).

Run-on: Mary laughed she went outside.

Correction: Mary laughed.  She went inside.

b.      Place a semi-colon between the two independent clauses.

Run-on: Mary laughed she went outside.

Correction: Mary laughed.  She went inside.

c.      Place a comma and a subordinating conjunction between the two independent clauses

Run-on: Mary laughed she went outside.

Correction: Mary laughed, and she went inside.

d.     Place a colon between the two independent clauses if the relationship of the second clarifies the first.

Run-on: Mary laughed she went outside.

Correction: Mary is our neighbor: she lives right next to us.

e.      Place a semi-colon or start a new sentence before the transitional expressions: the conjunctive adverbs, such as however, moreover, nevertheless.

Run-on: Mary won the race, however, she paid a very high price for that.

Correction: Mary won the race; however, she paid a very high price for that. 

f.        Adding dependent word.

Run-on: Mary bought a new car it was priced at $7,000.

Correction: Mary bought a new car which was priced at $7,000.

g.      Reduce one independent clause to a verbal or verbal phrase.

Run-on:  I woke up early, I went for a walk.

Correction: Waking up early, I went for a walk.