Bsrhrki

documentclass[]article
usepackagetabularx
newcolumntypeh>hsize=.5hsizeX % half width of X 

begindocument


begintable
 begintabularxlinewidth X 
 hline
 Ontologies &
 Details &
 Limitations\
 hline

 textbfccsla, textbfccpricing, textbfccinstances, textbfccregions,textbfdmcc-schema, textbfccdm.
 &
 Features:
 SLA, Price, VM instance feature and Region, Vendor. Data mining experiments parameters. 
 newlinenewline
 Availability:
 Schemas and data are online. Each schema has one or two examples modelling Services from Amazon. One exception is textbfccdm which has 6 examples of different ML experiments.
 &
 textbfccpricing can not handle the complexity of most common price options, like cost of OS to be installed on the VM, network data transfer cost differed by destination and usage, snapshot storage costs and etc. textbfccinstances does not allow unit to be specified in data, has to be fixed for each property.
 \
 hline
 endtabularx
 captionOntologies Comparison
 labeltable:OntologiesComparison
endtable

enddocument

If you want that the first column is half of the other two, you need that the sum of the fractions is 3, so the first column should be set at 3/5, and the other two at 6/5.

documentclass[]article
usepackagetabularx,booktabs

newcolumntypeY[1]>hsize=#1hsizeX

begindocument

begintable

begintabularxlinewidth@ >raggedrightY0.6 Y1.2 Y1.2 @
toprule
Ontologies & Details & Limitations\
midrule

textbfccsla, textbfccpricing, textbfccinstances, textbfccregions,
textbfdmcc-schema, textbfccdm.
&
Features:
SLA, Price, VM instance feature and Region, Vendor. Data mining experiments 
parameters.

bigskip

Availability:
Schemas and data are online. Each schema has one or two examples modelling 
Services from Amazon. One exception is textbfccdm which has 6 examples 
of different ML experiments.
&
textbfccpricing can not handle the complexity of most common price options, 
like cost of OS to be installed on the VM, network data transfer cost differed 
by destination and usage, snapshot storage costs and etc. textbfccinstances 
does not allow unit to be specified in data, has to be fixed for each property.
\
bottomrule
endtabularx

captionOntologies Comparison
labeltable:OntologiesComparison

endtable

enddocument

I added booktabs rules, that have more room around them, and typeset the first column ragged right.

As an alternative, after seeing that dmcc-schema gets broken, set the width accordingly.

documentclass[]article
usepackagetabularx,booktabs

newlengthmytabwidth

begindocument

begintable

settowidthmytabwidthtextbfdmcc-schema,

begintabularxlinewidth@ >raggedrightpmytabwidth X X @
toprule
Ontologies & Details & Limitations\
midrule

textbfccsla, textbfccpricing, textbfccinstances, textbfccregions,
textbfdmcc-schema, textbfccdm.
&
Features:
SLA, Price, VM instance feature and Region, Vendor. Data mining experiments 
parameters.

bigskip

Availability:
Schemas and data are online. Each schema has one or two examples modelling 
Services from Amazon. One exception is textbfccdm which has 6 examples 
of different ML experiments.
&
textbfccpricing can not handle the complexity of most common price options, 
like cost of OS to be installed on the VM, network data transfer cost differed 
by destination and usage, snapshot storage costs and etc. textbfccinstances 
does not allow unit to be specified in data, has to be fixed for each property.
\
bottomrule
endtabularx

captionOntologies Comparison
labeltable:OntologiesComparison

endtable

enddocument

From the tabularx documentation:

Make sure that the sum of the widths of all the Xcolumns is unchanged.[...]

You will therefore have to change from X column to >hsize=.5hsizeX for the second and third column (as 1.25+1.25+0.5=3 with a total number of 3 X type columns)

documentclass[]article
usepackagetabularx
newcolumntypeh>hsize=.5hsizeX % half width of X 

begindocument


begintable
 begintabularxlinewidth >hsize=.5hsizeX 
 hline
 Ontologies &
 Details &
 Limitations\
 hline

 textbfccsla, textbfccpricing, textbfccinstances, textbfccregions,textbfdmcc-schema, textbfccdm.
 &
 Features:
 SLA, Price, VM instance feature and Region, Vendor. Data mining experiments parameters. 
 newlinenewline
 Availability:
 Schemas and data are online. Each schema has one or two examples modelling Services from Amazon. One exception is textbfccdm which has 6 examples of different ML experiments.
 &
 textbfccpricing can not handle the complexity of most common price options, like cost of OS to be installed on the VM, network data transfer cost differed by destination and usage, snapshot storage costs and etc. textbfccinstances does not allow unit to be specified in data, has to be fixed for each property.
 \
 hline
 endtabularx
 captionOntologies Comparison
 labeltable:OntologiesComparison
endtable

enddocument

From The LaTeX Companion

Normally, all X columns in a single table are set to the same width.
It is nevertheless possible to make tabularx set them to different
widths. A preamble like the following

>setlengthhsize.5hsizeX>setlengthhsize1.5hsizeX}

specifies two columns; the second column will be three times as wide as
the first. However, when using this method two rules should be obeyed:

1. The sum of the widths of all X columns should remain unchanged. In
   the above example, the new widths should add up to the width of two
   standard X columns.


2. Any multicolumn entries that cross any X column should not be used.

Thus, the total summation of all factors attached to hsize must stay equal to the number of columns. In your case, you have 3 columns. Since column h is 0.5hsize, the other two must add to 3 (total number of columns) - 0.5 = 2.5. Dividing 2.5 over two gives us 1.25. Thus, the size for the other two columns should be hsize=1.25hsize

documentclass[]article
usepackagetabularx
newcolumntypeh>hsize=.5hsizeX % half width of X 

begindocument


begintable
 begintabularxlinewidth h 
 hline
 Ontologies &
 Details &
 Limitations\
 hline

 textbfccsla, textbfccpricing, textbfccinstances, textbfccregions,textbfdmcc-schema, textbfccdm.
 &
 Features:
 SLA, Price, VM instance feature and Region, Vendor. Data mining experiments parameters. 
 newlinenewline
 Availability:
 Schemas and data are online. Each schema has one or two examples modelling Services from Amazon. One exception is textbfccdm which has 6 examples of different ML experiments.
 &
 textbfccpricing can not handle the complexity of most common price options, like cost of OS to be installed on the VM, network data transfer cost differed by destination and usage, snapshot storage costs and etc. textbfccinstances does not allow unit to be specified in data, has to be fixed for each property.
 \
 hline
 endtabularx
 captionOntologies Comparison
 labeltable:OntologiesComparison
endtable

enddocument